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From Wikipedia, the free encyclopedia

Light is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye (about 400–700 nm, or perhaps 380–750 nm[1]). In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.[2][3]

Four primary properties of light are:

Light, which exists in tiny "packets" called photons, exhibits properties of both waves and particles. This property is referred to as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.


Speed of light

The speed of light in a vacuum is presently defined to be exactly 299,792,458 m/s (approximately 186,282 miles per second). This definition of the speed of light means that the metre is now defined in terms of the speed of light. Light always travels at a constant speed, even between particles of a substance through which it is shining. Photons excite the adjoining particles that in turn transfer the energy to the neighbor. This may appear to slow the beam down through its trajectory in realtime. The time lost between entry and exit accounts to the displacement of energy through the substance between each particle that is excited.

Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Ole observed the motions of Jupiter and one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, Rømer calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit.[4] Unfortunately, its size was not known at that time. If Ole had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s.

Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several kilometers away. A rotating cog wheel was placed in the path of the light beam as it traveled from the source, to the mirror and then returned to its origin. Fizeau found that at a certain rate of rotation, the beam would pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau was able to calculate the speed of light as 313,000,000 m/s.

Léon Foucault used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 299,796,000 m/s.

Two independent teams of physicists were able to bring light to a complete standstill by passing it through a Bose-Einstein Condensate of the element rubidium, one led by Dr. Lene Vestergaard Hau of Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other by Dr. Ronald L. Walsworth and Dr. Mikhail D. Lukin of the Harvard-Smithsonian Center for Astrophysics, also in Cambridge.[citation needed]

Electromagnetic spectrum

Electromagnetic spectrum with light highlighted

Generally, EM radiation (the designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays.

The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries.


Main article: Refraction

Refraction is the bending of light rays when passing from one transparent material to another. It is described by Snell's Law:

n_1\sin\theta_1 = n_2\sin\theta_2\ .

where θ1 is the angle between the ray and the normal in the first medium, θ2 is the angle between the ray and the normal in the second medium, and n1 and n2 are the indices of refraction, n = 1 in a vacuum and n > 1 in a transparent substance.

When a beam of light crosses the boundary between a vacuum and another medium, or between two different media, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not orthogonal (or rather normal) to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as refraction.

The refractive quality of lenses is frequently used to manipulate light in order to change the apparent size of images. Magnifying glasses, spectacles, contact lenses, microscopes and refracting telescopes are all examples of this manipulation.

Light refraction is the main basis of measurement for gloss. Gloss is measured using a glossmeter.


The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows and the aurora borealis offer many clues as to the nature of light as well as much enjoyment.

Light sources

A cloud illuminated by sunlight

There are many sources of light. The most common light sources are thermal: a body at a given temperature emits a characteristic spectrum of black-body radiation. Examples include sunlight (the radiation emitted by the chromosphere of the Sun at around 6,000 K peaks in the visible region of the electromagnetic spectrum when plotted in wavelength units [1] and roughly 40% of sunlight is visible), incandescent light bulbs (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in flames. The peak of the blackbody spectrum is in the infrared for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue color as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colors can be seen when metal is heated to "red hot" or "white hot". Blue thermal emission is not often seen. The commonly seen blue colour in a gas flame or a welder's torch is in fact due to molecular emission, notably by CH radicals (emitting a wavelength band around 425 nm).

Atoms emit and absorb light at characteristic energies. This produces "emission lines" in the spectrum of each atom. Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames (light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light). Emission can also be stimulated, as in a laser or a microwave maser.

Deceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation.

Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake.

Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence.

Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example of this. This mechanism is used in cathode ray tube television sets and computer monitors.

A city illuminated by light bulbs

Certain other mechanisms can produce light:

When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include:

Units and measures

Light is measured with two main alternative sets of units: radiometry consists of measurements of light power at all wavelengths, while photometry measures light with wavelength weighted with respect to a standardized model of human brightness perception. Photometry is useful, for example, to quantify illumination intended for human use. The SI units for both systems are summarized in the following tables.


SI radiometry units
Quantity Symbol SI unit Abbr. Notes
Radiant energy Q joule J energy
Radiant flux Φ watt W radiant energy per unit time, also called radiant power
Radiant intensity I watt per steradian W·sr−1 power per unit solid angle
Radiance L watt per steradian per square metre W·sr−1·m−2 power per unit solid angle per unit projected source area.

called intensity in some other fields of study.

Irradiance E, I watt per square metre W·m−2 power incident on a surface.

sometimes confusingly called "intensity".

Radiant exitance /
Radiant emittance
M watt per square metre W·m−2 power emitted from a surface.
Radiosity J or Jλ watt per square metre W·m−2 emitted plus reflected power leaving a surface
Spectral radiance Lλ
watt per steradian per metre3

watt per steradian per square
metre per hertz



commonly measured in W·sr−1·m−2·nm−1

Spectral irradiance Eλ
watt per metre3
watt per square metre per hertz
commonly measured in W·m−2·nm−1

SI photometry units
Quantity Symbol SI unit Abbr. Notes
Luminous energy Qv lumen second lm·s units are sometimes called talbots
Luminous flux F lumen (= cd·sr) lm also called luminous power
Luminous intensity Iv candela (= lm/sr) cd an SI base unit
Luminance Lv candela per square metre cd/m2 units are sometimes called "nits"
Illuminance Ev lux (= lm/m2) lx Used for light incident on a surface
Luminous emittance Mv lux (= lm/m2) lx Used for light emitted from a surface
Luminous efficacy   lumen per watt lm/W ratio of luminous flux to radiant flux
See also SI · Photometry

The photometry units are different from most systems of physical units in that they take into account how the human eye responds to light. The cone cells in the human eye are of three types which respond differently across the visible spectrum, and the cumulative response peaks at a wavelength of around 555 nm. Therefore, two sources of light which produce the same intensity (W/m2) of visible light do not necessarily appear equally bright. The photometry units are designed to take this into account, and therefore are a better representation of how "bright" a light appears to be than raw intensity. They relate to raw power by a quantity called luminous efficacy, and are used for purposes like determining how to best achieve sufficient illumination for various tasks in indoor and outdoor settings. The illumination measured by a photocell sensor does not necessarily correspond to what is perceived by the human eye, and without filters which may be costly, photocells and CCDs tend to respond to some infrared, ultraviolet or both.

Historical theories about light, in chronological order

Hindu and Buddhist theories

In ancient India, the Hindu schools of Samkhya and Vaisheshika, from around the 6th–5th century BC, developed theories on light. According to the Samkhya school, light is one of the five fundamental "subtle" elements (tanmatra) out of which emerge the gross elements. The atomicity of these elements is not specifically mentioned and it appears that they were actually taken to be continuous.

On the other hand, the Vaisheshika school gives an atomic theory of the physical world on the non-atomic ground of ether, space and time. (See Indian atomism.) The basic atoms are those of earth (prthivı), water (pani), fire (agni), and air (vayu), that should not be confused with the ordinary meaning of these terms. These atoms are taken to form binary molecules that combine further to form larger molecules. Motion is defined in terms of the movement of the physical atoms and it appears that it is taken to be non-instantaneous. Light rays are taken to be a stream of high velocity of tejas (fire) atoms. The particles of light can exhibit different characteristics depending on the speed and the arrangements of the tejas atoms. Around the first century BC, the Vishnu Purana refers to sunlight as the "the seven rays of the sun".

Later in 499, Aryabhata, who proposed a heliocentric solar system of gravitation in his Aryabhatiya, wrote that the planets and the Moon do not have their own light but reflect the light of the Sun.

The Indian Buddhists, such as Dignāga in the 5th century and Dharmakirti in the 7th century, developed a type of atomism that is a philosophy about reality being composed of atomic entities that are momentary flashes of light or energy. They viewed light as being an atomic entity equivalent to energy, similar to the modern concept of photons, though they also viewed all matter as being composed of these light/energy particles.

It is written in the Rigveda that light consists of three primary colors. "Mixing the three colours, ye have produced all the objects of sight!"[5]

Greek and Hellenistic theories

In the fifth century BC, Empedocles postulated that everything was composed of four elements; fire, air, earth and water. He believed that Aphrodite made the human eye out of the four elements and that she lit the fire in the eye which shone out from the eye making sight possible. If this were true, then one could see during the night just as well as during the day, so Empedocles postulated an interaction between rays from the eyes and rays from a source such as the sun.

In about 300 BC, Euclid wrote Optica, in which he studied the properties of light. Euclid postulated that light travelled in straight lines and he described the laws of reflection and studied them mathematically. He questioned that sight is the result of a beam from the eye, for he asks how one sees the stars immediately, if one closes one's eyes, then opens them at night. Of course if the beam from the eye travels infinitely fast this is not a problem.

In 55 BC, Lucretius, a Roman who carried on the ideas of earlier Greek atomists, wrote:

"The light & heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove." – On the nature of the Universe

Despite being similar to later particle theories, Lucretius's views were not generally accepted and light was still theorized as emanating from the eye.

Ptolemy (c. 2nd century) wrote about the refraction of light in his book Optics, and developed a theory of vision whereby objects are seen by rays of light emanating from the eyes.[6]

Optical theory

Ibn al-Haytham proved that light travels in straight lines through optical experiments.

The Muslim scientist, Ibn al-Haytham (965–1040), known as Alhacen or Alhazen in the West, developed a broad theory of vision based on geometry and anatomy in his Book of Optics (1021). Ibn al-Haytham provided the first correct description of how vision works,[7] explaining that it is not due to objects being seen by rays of light emanating from the eyes, as Euclid and Ptolemy had assumed, but due to light rays entering the eyes.[8] Ibn al-Haytham postulated that every point on an illuminated surface radiates light rays in all directions, but that only one ray from each point can be seen: the ray that strikes the eye perpendicularly. The other rays strike at different angles and are not seen. He conducted experiments to support his argument, which included the development of apparatus such as the pinhole camera and camera obscura, which produces an inverted image.[9] Alhacen held light rays to be streams of minute particles that "lack all sensible qualities except energy"[10] and travel at a finite speed.[11][12][13] He improved Ptolemy's theory of the refraction of light, and went on to describe the laws of refraction, though this was earlier discovered by Ibn Sahl (c. 940-1000) several decades before him.[14][15]

A page of Ibn Sahl's manuscript showing his discovery of the law of refraction (Snell's law).

He also carried out the first experiments on the dispersion of light into its constituent colors. His major work Kitab al-Manazir (Book of Optics) was translated into Latin in the Middle Ages, as well his book dealing with the colors of sunset. He dealt at length with the theory of various physical phenomena like shadows, eclipses, the rainbow. He also attempted to explain binocular vision, and gave an explanation of the apparent increase in size of the sun and the moon when near the horizon, known as the moon illusion. Because of his extensive experimental research on optics, Ibn al-Haytham is considered the "father of modern optics".[16]

Ibn al-Haytham developed the camera obscura and pinhole camera for his experiments on light.

Ibn al-Haytham also correctly argued that we see objects because the sun's rays of light, which he believed to be streams of tiny energy particles[10] travelling in straight lines, are reflected from objects into our eyes.[11] He understood that light must travel at a large but finite velocity,[11][12][13] and that refraction is caused by the velocity being different in different substances.[11] He also studied spherical and parabolic mirrors, and understood how refraction by a lens will allow images to be focused and magnification to take place. He understood mathematically why a spherical mirror produces aberration.

Ibn al-Haytham's optical model of light was "the first comprehensive and systematic alternative to Greek optical theories."[17] He initiated a revolution in optics and visual perception,[18][19][20][21][22][23] also known as the 'Optical Revolution',[24] and laid the foundations for a physical optics.[25][26] As such, he is often regarded as the "father of modern optics."[25]

Avicenna (980–1037) agreed that the speed of light is finite, as he "observed that if the perception of light is due to the emission of some sort of particles by a luminous source, the speed of light must be finite."[27] Abū Rayhān al-Bīrūnī (973–1048) also agreed that light has a finite speed, and he was the first to discover that the speed of light is much faster than the speed of sound.[28] In the late 13th and early 14th centuries, Qutb al-Din al-Shirazi (1236–1311) and his student Kamāl al-Dīn al-Fārisī (1260–1320) continued the work of Ibn al-Haytham, and they were the first to give the correct explanations for the rainbow phenomenon.[29]

René Descartes (1596–1650) held that light was a mechanical property of the luminous body, rejecting the "forms" of Ibn al-Haytham and Whitelo as well as the "species" of Bacon, Grosseteste, and Kepler.[30] In 1637 he published a theory of the refraction of light that assumed, incorrectly, that light travelled faster in a denser medium than in a less dense medium. Descartes arrived at this conclusion by analogy with the behaviour of sound waves.[citation needed] Although Descartes was incorrect about the relative speeds, he was correct in assuming that light behaved like a wave and in concluding that refraction could be explained by the speed of light in different media.

Descartes is not he first to use the mechanical analogies but because he clearly asserts that light is only a mechanical property of the luminous body and the transmitting medium, Descartes' theory of light is regarded as start of modern physical optics.[31]

Particle theory

Ibn al-Haytham (Alhazen, 965–1040) proposed a particle theory of light in his Book of Optics (1021). He held light rays to be streams of minute energy particles[10] that travel in straight lines at a finite speed.[11][12][13] He states in his optics that "the smallest parts of light," as he calls them, "retain only properties that can be treated by geometry and verified by experiment; they lack all sensible qualities except energy."[10] Avicenna (980–1037) also proposed that "the perception of light is due to the emission of some sort of particles by a luminous source".[27]

Pierre Gassendi (1592–1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s. Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the plenum. He stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines. He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether.

Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to hold sway during the 18th century. The particle theory of light led Laplace to argue that a body could be so massive that light could not escape from it. In other words it would become what is now called a black hole. Laplace withdrew his suggestion when the wave theory of light was firmly established. A translation of his essay appears in The large scale structure of space-time, by Stephen Hawking and George F. R. Ellis.

Wave theory

In the 1660s, Robert Hooke published a wave theory of light. Christiaan Huygens worked out his own wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the Luminiferous ether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.

Thomas Young's sketch of the two-slit experiment showing the diffraction of light. Young's experiments supported the theory that light consists of waves.

The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 by Thomas Young), and that light could be polarized, if it were a transverse wave. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colors were caused by different wavelengths of light, and explained color vision in terms of three-colored receptors in the eye.

Another supporter of the wave theory was Leonhard Euler. He argued in Nova theoria lucis et colorum (1746) that diffraction could more easily be explained by a wave theory.

Later, Augustin-Jean Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817. Simeon Denis Poisson added to Fresnel's mathematical work to produce a convincing argument in favour of the wave theory, helping to overturn Newton's corpuscular theory. By the year 1821, Fresnel was able to show via mathematical methods that polarization could be explained only by the wave theory of light and only if light was entirely transverse, with no longitudinal vibration whatsoever.

The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. A hypothetical substance called the luminiferous aether was proposed, but its existence was cast into strong doubt in the late nineteenth century by the Michelson-Morley experiment.

Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the speed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was Léon Foucault, in 1850.[32] His result supported the wave theory, and the classical particle theory was finally abandoned.

Electromagnetic theory

A linearly-polarized light wave frozen in time and showing the two oscillating components of light; an electric field and a magnetic field perpendicular to each other and to the direction of motion (a transverse wave).

In 1845, Michael Faraday discovered that the plane of polarization of linearly polarized light is rotated when the light rays travel along the magnetic field direction in the presence of a transparent dielectric, an effect now known as Faraday rotation.[33] This was the first evidence that light was related to electromagnetism. In 1846 he speculated that light might be some form of disturbance propagating along magnetic field lines.[34] Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the ether.

Faraday's work inspired James Clerk Maxwell to study electromagnetic radiation and light. Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation: he first stated this result in 1862 in On Physical Lines of Force. In 1873, he published A Treatise on Electricity and Magnetism, which contained a full mathematical description of the behaviour of electric and magnetic fields, still known as Maxwell's equations. Soon after, Heinrich Hertz confirmed Maxwell's theory experimentally by generating and detecting radio waves in the laboratory, and demonstrating that these waves behaved exactly like visible light, exhibiting properties such as reflection, refraction, diffraction, and interference. Maxwell's theory and Hertz's experiments led directly to the development of modern radio, radar, television, electromagnetic imaging, and wireless communications.

The special theory of relativity

The wave theory was wildly successful in explaining nearly all optical and electromagnetic phenomena, and was a great triumph of nineteenth century physics. By the late nineteenth century, however, a handful of experimental anomalies remained that could not be explained by or were in direct conflict with the wave theory. One of these anomalies involved a controversy over the speed of light. The constant speed of light predicted by Maxwell's equations and confirmed by the Michelson-Morley experiment contradicted the mechanical laws of motion that had been unchallenged since the time of Galileo, which stated that all speeds were relative to the speed of the observer. In 1905, Albert Einstein resolved this paradox by revising the Galilean model of space and time to account for the constancy of the speed of light. Einstein formulated his ideas in his special theory of relativity, which advanced humankind's understanding of space and time. Einstein also demonstrated a previously unknown fundamental equivalence between energy and mass with his famous equation

E = mc^2 \,

where E is energy, m is, depending on the context, the rest mass or the relativistic mass, and c is the speed of light in a vacuum.

Particle theory revisited

Another experimental anomaly was the photoelectric effect, by which light striking a metal surface ejected electrons from the surface, causing an electric current to flow across an applied voltage. Experimental measurements demonstrated that the energy of individual ejected electrons was proportional to the frequency, rather than the intensity, of the light. Furthermore, below a certain minimum frequency, which depended on the particular metal, no current would flow regardless of the intensity. These observations appeared to contradict the wave theory, and for years physicists tried in vain to find an explanation. In 1905, Einstein solved this puzzle as well, this time by resurrecting the particle theory of light to explain the observed effect. Because of the preponderance of evidence in favor of the wave theory, however, Einstein's ideas were met initially by great skepticism among established physicists. But eventually Einstein's explanation of the photoelectric effect would triumph, and it ultimately formed the basis for wave–particle duality and much of quantum mechanics.

Quantum theory

A third anomaly that arose in the late 19th century involved a contradiction between the wave theory of light and measurements of the electromagnetic spectrum emitted by thermal radiators, or so-called black bodies. Physicists struggled with this problem, which later became known as the ultraviolet catastrophe, unsuccessfully for many years. In 1900, Max Planck developed a new theory of black-body radiation that explained the observed spectrum correctly. Planck's theory was based on the idea that black bodies emit light (and other electromagnetic radiation) only as discrete bundles or packets of energy. These packets were called quanta, and the particle of light was given the name photon, to correspond with other particles being described around this time, such as the electron and proton. A photon has an energy, E, proportional to its frequency, f, by

E = hf = \frac{hc}{\lambda} \,\!

where h is Planck's constant, λ is the wavelength and c is the speed of light. Likewise, the momentum p of a photon is also proportional to its frequency and inversely proportional to its wavelength:

p = { E \over c } = { hf \over c } = { h \over \lambda }.

As it originally stood, this theory did not explain the simultaneous wave- and particle-like natures of light, though Planck would later work on theories that did. In 1918, Planck received the Nobel Prize in Physics for his part in the founding of quantum theory.

Wave–particle duality

The modern theory that explains the nature of light includes the notion of wave–particle duality, described by Albert Einstein in the early 1900s, based on his study of the photoelectric effect and Planck's results. Einstein asserted that the energy of a photon is proportional to its frequency. More generally, the theory states that everything has both a particle nature and a wave nature, and various experiments can be done to bring out one or the other. The particle nature is more easily discerned if an object has a large mass, and it was not until a bold proposition by Louis de Broglie in 1924 that the scientific community realized that electrons also exhibited wave–particle duality. The wave nature of electrons was experimentally demonstrated by Davisson and Germer in 1927. Einstein received the Nobel Prize in 1921 for his work with the wave–particle duality on photons (especially explaining the photoelectric effect thereby), and de Broglie followed in 1929 for his extension to other particles.

Quantum electrodynamics

The quantum mechanical theory of light and electromagnetic radiation continued to evolve through the 1920s and 1930's, and culminated with the development during the 1940s of the theory of quantum electrodynamics, or QED. This so-called quantum field theory is among the most comprehensive and experimentally successful theories ever formulated to explain a set of natural phenomena. QED was developed primarily by physicists Richard Feynman, Freeman Dyson, Julian Schwinger, and Shin-Ichiro Tomonaga. Feynman, Schwinger, and Tomonaga shared the 1965 Nobel Prize in Physics for their contributions.

Light pressure

Light pushes on objects in its path, just as the wind would do. This pressure is most easily explainable in particle theory: photons hit and transfer their momentum. Light pressure can cause asteroids to spin faster,[35] acting on their irregular shapes as on the vanes of a windmill. The possibility to make solar sails that would accelerate spaceships in space is also under investigation.[36][37]

Although the motion of the Crookes radiometer was originally attributed to light pressure, this interpretation is incorrect; the characteristic Crookes rotation is the result of a partial vacuum.[38] This should not be confused with the Nichols radiometer, in which the motion is directly caused by light pressure.[39]


An intricate display for the feast of St. Thomas at Kallara Pazhayapalli in Kottayam, Kerala, India dramatically illustrates the importance of light in religion.

The sensory perception of light plays a central role in spirituality (vision, enlightenment, darshan, Tabor Light). The presence of light as opposed to its absence (darkness) is a common metaphor of good and evil, knowledge and ignorance, and similar concepts. This idea is prevalent in both Eastern and Western spirituality.

See also


  1. ^ Karen E. Kalumuck (2000). Human body explorations: hands-on investigates of what makes us tick. Kendall/Hunt Publishing Company. p. 74. ISBN 9780787261535. 
  2. ^ Gregory Hallock Smith (2006). Camera lenses: from box camera to digital. SPIE Press. p. 4. ISBN 9780819460936. 
  3. ^ Narinder Kumar (2008). Comprehensive Physics XII. Laxmi Publications. p. 1416. ISBN 9788170085928. 
  4. ^ Scientific Method, Statistical Method and the Speed of Light. Statistical Science 2000, Vol. 15, No. 3, 254–278
  5. ^ Vyasa, Krishna-Dwai, The Mahabharata of Krishna-Dwaipayana Vyasa First Book Adi Parva, The Echo Library, p. 41, ISBN 978-1-40687-045-9, , Section III , p. 41
  6. ^ Ptolemy and A. Mark Smith (1996). Ptolemy's Theory of Visual Perception: An English Translation of the Optics with Introduction and Commentary. Diane Publishing. p. 23. ISBN 0-871-69862-5. 
  7. ^ Bashar Saad, Hassan Azaizeh, Omar Said (October 2005). "Tradition and Perspectives of Arab Herbal Medicine: A Review", Evidence-based Complementary and Alternative Medicine 2 (4), p. 475-479 [476]. Oxford University Press.
  8. ^ D. C. Lindberg, Theories of Vision from al-Kindi to Kepler, (Chicago, Univ. of Chicago Pr., 1976), pp. 60-7.
  9. ^ History of Photography and the Camera
  10. ^ a b c d Rashed, Roshdi (2007), "The Celestial Kinematics of Ibn al-Haytham", Arabic Sciences and Philosophy (Cambridge University Press) 17: 7–55 [19], doi:10.1017/S0957423907000355 :
    "In his optics ‘‘the smallest parts of light’’, as he calls them, retain only properties that can be treated by geometry and verified by experiment; they lack all sensible qualities except energy."
  11. ^ a b c d e O'Connor, John J.; Robertson, Edmund F., "Abu Ali al-Hasan ibn al-Haytham", MacTutor History of Mathematics archive, University of St Andrews, .
  12. ^ a b c MacKay, R. J.; Oldford, R. W. (August 2000), "Scientific Method, Statistical Method and the Speed of Light", Statistical Science 15 (3): 254–78, doi:10.1214/ss/1009212817 
  13. ^ a b c Sami Hamarneh (March 1972). Review of Hakim Mohammed Said, Ibn al-Haitham, Isis 63 (1), p. 119.
  14. ^ K. B. Wolf, "Geometry and dynamics in refracting systems", European Journal of Physics 16, p. 14-20, 1995.
  15. ^ R. Rashed, "A pioneer in anaclastics: Ibn Sahl on burning mirrors and lenses", Isis 81, p. 464–491, 1990.
  16. ^ R. L. Verma (1969). Al-Hazen: father of modern optics.
  17. ^ D. C. Lindberg, "Alhazen's Theory of Vision and its Reception in the West", Isis, 58 (1967), p. 322.
  18. ^ Sabra, A. I.; Hogendijk, J. P. (2003), The Enterprise of Science in Islam: New Perspectives, MIT Press, pp. 85–118, ISBN 0262194821, OCLC 237875424 
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Up to date as of January 14, 2010

From Wikiquote

And God said: "Let there be light," and there was light. ~ The Bible
Light upon light, God guideth unto His light whome he will. And God speaketh to mankind in allegories, for God is knower of all things. ~ Quran

Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, the word is sometimes used to mean electromagnetic radiation of all wavelengths. The elementary particle that defines light is the photon.

The sensory perception of light plays a central role in Spirituality, and the presence of light as opposed to its absence (darkness) is a common Western metaphor of good and evil, knowledge and ignorance, and similar concepts.


  • And God said: "Let there be light," and there was light.
  • God is the light of the heavens and the earth. The similitude of His light is as a niche wherein is a lamp. The lamp is in a glass.The glass is as it were a shining star. (This lamp is) kindled from a blessed tree, an olive neither of the East nor of the West, whose oil would almost glow forth (of itself) though no fire touched it. Light upon light, God guideth unto His light whome he will. And God speaketh to mankind in allegories, for God is knower of all things.
  • Hail, holy light! offspring of heaven first-born.
  • Light travels faster than sound, but sound is still louder.
  • There are two kinds of light — the glow that illumines, and the glare that obscures.


  • Enlighten the path of others – darkness lights its own way.
    • Leonid S. Sukhorukov
  • Light itself is a great corrective. A thousand wrongs and abuses that are grown in darkness disappear, like owls and bats, before the light of day.
  • More light!
  • No wonder that light is so frequently used by the sacred oracles as the symbol of our best blessings. Of the Gospel revelation one apostle says, "The night is far spent, and the day is at hand." Another, under the impression of the same auspicious event, thus applied the language of ancient prophecy: "The people who sat in darkness have seen a great light; and to them which sat in the region and shadow of death light is sprung up."
    • Baseley
  • The light in the world comes principally from two sources,—the sun, and the student's lamp.
    • Christian Nestell Bovee
  • We should render thanks to God for having produced this temporal light, which is the smile of heaven and joy of the world, spreading it like a cloth of gold over the face of the air and earth, and lighting it as a torch by which we might behold His works.
    • Caussin

External links

Wikipedia has an article about:
Look up light in Wiktionary, the free dictionary

Source material

Up to date as of January 22, 2010

From Wikisource

by John Ruskin

146. The plan of the divisions of art-schools which I gave you in the last lecture is of course only a first germ of classification, on which we are to found farther and more defined statement; but for this very reason it is necessary that every term of it should be very clear in your minds.

And especially I must explain, and ask you to note the sense in which I use the word "mass." Artists usually employ that word to express the spaces of light and darkness, or of colour, into which a picture is divided. But this habit of theirs arises partly from their always speaking of pictures in which the lights represent solid form. If they had instead been speaking of flat tints, as, for instance, of the gold and blue in this missal page, they would not have called them "masses," but "spaces" of colour. Now both for accuracy and convenience' sake, you will find it well to observe this distinction, and to call a simple flat tint a space of colour; and only the representation of solid or projecting form a mass.

I use, however, the word "line" rather than "space" in the second and third heads of our general scheme, at p. 94, because you cannot limit a flat tint but by a line, or the locus of a line: whereas a gradated tint, expressive of mass, may be lost at its edges in another, without any fixed limit; and practically is so, in the works of the greatest masters.

147. You have thus, in your hexagonal scheme, the expression of the universal manner of advance in painting: Line first; then line enclosing flat spaces coloured or shaded; then the lines vanish, and the solid forms are seen within the spaces. That is the universal law of advance:--1, line; 2, flat space; 3, massed or solid space. But as you see, this advance may be made, and has been made, by two different roads; one advancing always through colour, the other through light and shade. And these two roads are taken by two entirely different kinds of men. The way by colour is taken by men of cheerful, natural, and entirely sane disposition in body and mind, much resembling, even at its strongest, the temper of well-brought-up children:--too happy to think deeply, yet with powers of imagination by which they can live other lives than their actual ones: make-believe lives, while yet they remain conscious all the while that they _are_ making believe--therefore entirely sane. They are also absolutely contented; they ask for no more light than is immediately around them, and cannot see anything like darkness, but only green and blue, in the earth and sea.

148. The way by light and shade is, on the contrary, taken by men of the highest powers of thought, and most earnest desire for truth; they long for light, and for knowledge of all that light can show. But seeking for light, they perceive also darkness; seeking for truth and substance, they find vanity. They look for form in the earth,--for dawn in the sky; and seeking these, they find formlessness in the earth, and night in the sky.

Now remember, in these introductory lectures I am putting before you the roots of things, which are strange, and dark, and often, it may seem, unconnected with the branches. You may not at present think these metaphysical statements necessary; but as you go on, you will find that having hold of the clue to methods of work through their springs in human character, you may perceive unerringly where they lead, and what constitutes their wrongness and rightness; and when we have the main principles laid down, all others will develop themselves in due succession, and everything will become more clearly intelligible to you in the end, for having been apparently vague in the beginning. You know when one is laying the foundation of a house, it does not show directly where the rooms are to be.

149. You have then these two great divisions of human mind: one, content with the colours of things, whether they are dark or light; the other seeking light pure, as such, and dreading darkness as such. One, also, content with the coloured aspects and visionary shapes of things; the other seeking their form and substance. And, as I said, the school of knowledge, seeking light, perceives, and has to accept and deal with obscurity: and seeking form, it has to accept and deal with formlessness, or death.

Farther, the school of colour in Europe, using the word Gothic in its broadest sense, is essentially Gothic _Christian_; and full of comfort and peace. Again, the school of light is essentially Greek, and full of sorrow. I cannot tell you which is right, or least wrong. I tell you only what I know--this vital distinction between them: the Gothic or colour school is always cheerful, the Greek always oppressed by the shadow of death; and the stronger its masters are, the closer that body of death grips them. The strongest whose work I can show you in recent periods is Holbein; next to him is Lionardo; and then Duerer: but of the three Holbein is the strongest, and with his help I will put the two schools in their full character before you in a moment.

150. Here is, first, the photograph of an entirely characteristic piece of the great colour school. It is by Cima of Conegliano, a mountaineer, like Luini, born under the Alps of Friuli. His Christian name was John Baptist: he is here painting his name-Saint; the whole picture full of peace, and intense faith and hope, and deep joy in light of sky, and fruit and flower and weed of earth. It was painted for the church of Our Lady of the Garden at Venice, La Madonna dell' Orto (properly Madonna of the _Kitchen_ Garden), and it is full of simple flowers, and has the wild strawberry of Cima's native mountains gleaming through the grass.

Beside it I will put a piece of the strongest work of the school of light and shade--strongest because Holbein was a colourist also; but he belongs, nevertheless, essentially to the chiaroscuro school. You know that his name is connected, in ideal work, chiefly with his "Dance of Death." I will not show you any of the terror of that; only a photograph of his well-known "Dead Christ." It will at once show you how completely the Christian art of this school is oppressed by its veracity, and forced to see what is fearful, even in what it most trusts.

You may think I am showing you contrasts merely to fit my theories. But there is Duerer's "Knight and Death," his greatest plate; and if I had Lionardo's "Medusa" here, which he painted when only a boy, you would have seen how he was held by the same chain. And you cannot but wonder why, this being the melancholy temper of the great Greek or naturalistic school, I should have called it the school of light. I call it so because it is through its intense love of light that the darkness becomes apparent to it, and through its intense love of truth and form that all mystery becomes attractive to it. And when, having learned these things, it is joined to the school of colour, you have the perfect, though always, as I will show you, pensive, art of Titian and his followers.

151. But remember, its first development, and all its final power, depend on Greek sorrow, and Greek religion.

The school of light is founded in the Doric worship of Apollo, and the Ionic worship of Athena, as the spirits of life in the light, and of life in the air, opposed each to their own contrary deity of death--Apollo to the Python, Athena to the Gorgon--Apollo as life in light, to the earth spirit of corruption in darkness;--Athena, as life by motion, to the Gorgon spirit of death by pause, freezing or turning to stone: both of the great divinities taking their glory from the evil they have conquered; both of them, when angry, taking to men the form of the evil which is their opposite--Apollo slaying by poisoned arrow, by pestilence; Athena by cold, the black aegis on her breast.

These are the definite and direct expressions of the Greek thoughts respecting death and life. But underlying both these, and far more mysterious, dreadful, and yet beautiful, there is the Greek conception of _spiritual_ darkness; of the anger of fate, whether foredoomed or avenging; the root and theme of all Greek tragedy; the anger of the Erinnyes, and Demeter Erinnys, compared to which the anger either of Apollo or Athena is temporary and partial:--and also, while Apollo or Athena only slay, the power of Demeter and the Eumenides is over the whole life; so that in the stories of Bellerophon, of Hippolytus, of Orestes, of Oedipus, you have an incomparably deeper shadow than any that was possible to the thought of later ages, when the hope of the Resurrection had become definite. And if you keep this in mind, you will find every name and legend of the oldest history become full of meaning to you. All the mythic accounts of Greek sculpture begin in the legends of the family of Tantalus. The main one is the making of the ivory shoulder of Pelops after Demeter has eaten the shoulder of flesh. With that you have Broteas, the brother of Pelops, carving the first statue of the mother of the gods; and you have his sister, Niobe, weeping herself to stone under the anger of the deities of light. Then Pelops himself, the dark-faced, gives name to the Peloponnesus, which you may therefore read as the "isle of darkness;" but its central city, Sparta, the "sown city," is connected with all the ideas of the earth as life-giving. And from her you have Helen, the representative of light in beauty, and the Fratres Helenae--"lucida sidera;" and, on the other side of the hills, the brightness of Argos, with its correlative darkness over the Atreidae, marked to you by Helios turning away his face from the feast of Thyestes.

152. Then join with these the Northern legends connected with the air. It does not matter whether you take Dorus as the son of Apollo or the son of Helen; he equally symbolises the power of light: while his brother, AEolus, through all his descendants, chiefly in Sisyphus, is confused or associated with the real god of the winds, and represents to you the power of the air. And then, as this conception enters into art, you have the myths of Daedalus, the flight of Icarus, and the story of Phrixus and Helle, giving you continual associations of the physical air and light, ending in the power of Athena over Corinth as well as over Athens.

Now, once having the clue, you can work out the sequels for yourselves better than I can for you; and you will soon find even the earliest or slightest grotesques of Greek art become full of interest. For nothing is more wonderful than the depth of meaning which nations in their first days of thought, like children, can attach to the rudest symbols; and what to us is grotesque or ugly, like a little child's doll, can speak to them the loveliest things. I have brought you to-day a few more examples of early Greek vase painting, respecting which remember generally that its finest development is for the most part sepulchral. You have, in the first period, always energy in the figures, light in the sky or upon the figures;[13] in the second period, while the conception of the divine power remains the same, it is thought of as in repose, and the light is in the god, not in the sky; in the time of decline, the divine power is gradually disbelieved, and all form and light are lost together. With that period I wish you to have nothing to do. You shall not have a single example of it set before you, but shall rather learn to recognise afterwards what is base by its strangeness. These, which are to come early in the third group of your Standard series, will enough represent to you the elements of early and late conception in the Greek mind of the deities of light.

[Footnote 13: See Note in the Catalogue on No. 201.]

153. First (S. 204), you have Apollo ascending from the sea; thought of as the physical sunrise: only a circle of light for his head; his chariot horses, seen foreshortened, black against the day-break, their feet not yet risen above the horizon. Underneath is the painting from the opposite side of the same vase: Athena as the morning breeze, and Hermes as the morning cloud, flying across the waves before the sunrise. At the distance I now hold them from you, it is scarcely possible for you to see that they are figures at all, so like are they to broken fragments of flying mist; and when you look close, you will see that as Apollo's face is invisible in the circle of light, Mercury's is invisible in the broken form of cloud: but I can tell you that it is conceived as reverted, looking back to Athena; the grotesque appearance of feature in the front is the outline of his hair.

These two paintings are excessively rude, and of the archaic period; the deities being yet thought of chiefly as physical powers in violent agency.

Underneath these two are Athena and Hermes, in the types attained about the time of Phidias; but, of course, rudely drawn on the vase, and still more rudely in this print from Le Normant and De Witte. For it is impossible (as you will soon find if you try for yourself) to give on a plane surface the grace of figures drawn on one of solid curvature, and adapted to all its curves: and among other minor differences, Athena's lance is in the original nearly twice as tall as herself, and has to be cut short to come into the print at all. Still, there is enough here to show you what I want you to see--the repose, and entirely realised personality, of the deities as conceived in the Phidian period. The relation of the two deities is, I believe, the same as in the painting above, though probably there is another added of more definite kind. But the physical meaning still remains--Athena unhelmeted, as the _gentle_ morning wind, commanding the cloud Hermes to slow flight. His petasus is slung at his back, meaning that the clouds are not yet opened or expanded in the sky.

154. Next (S. 205), you have Athena, again unhelmeted and crowned with leaves, walking between two nymphs, who are crowned also with leaves; and all the three hold flowers in their hands, and there is a fawn walking at Athena's feet.

This is still Athena as the morning air, but upon the earth instead of in the sky, with the nymphs of the dew beside her; the flowers and leaves opening as they breathe upon them. Note the white gleam of light on the fawn's breast; and compare it with the next following examples:--(underneath this one is the contest of Athena and Poseidon, which does not bear on our present subject).

Next (S. 206), Artemis as the moon of morning, walking low on the hills, and singing to her lyre; the fawn beside her, with the gleam of light and sunrise on its ear and breast. Those of you who are often out in the dawntime know that there is no moon so glorious as that gleaming crescent, though in its wane, ascending _before_ the sun.

Underneath, Artemis, and Apollo, of Phidian time.

Next (S. 207), Apollo walking on the earth, god of the morning, singing to his lyre; the fawn beside him, again with the gleam of light on its breast. And underneath, Apollo, crossing the sea to Delphi, of the Phidian time.

155. Now you cannot but be struck in these three examples with the similarity of action in Athena, Apollo, and Artemis, drawn as deities of the morning; and with the association in every case of the fawn with them. It has been said (I will not interrupt you with authorities) that the fawn belongs to Apollo and Diana because stags are sensitive to music; (are they?). But you see the fawn is here with Athena of the dew, though she has no lyre; and I have myself no doubt that in this particular relation to the gods of morning it always stands as the symbol of wavering and glancing motion on the ground, as well as of the light and shadow through the leaves, chequering the ground as the fawn is dappled. Similarly the spots on the nebris of Dionysus, thought of sometimes as stars (+apo tes ton astron poikilias+, Diodorus, I. 11), as well as those of his panthers, and the cloudings of the tortoise-shell of Hermes, are all significant of this light of the sky broken by cloud-shadow.

156. You observe also that in all the three examples the fawn has light on its ears, and face, as well as its breast. In the earliest Greek drawings of animals, bars of white are used as one means of detaching the figures from the ground; ordinarily on the under side of them, marking the lighter colour of the hair in wild animals. But the placing of this bar of white, or the direction of the face in deities of light, (the faces and flesh of women being always represented as white,) may become expressive of the direction of the light, when that direction is important. Thus we are enabled at once to read the intention of this Greek symbol of the course of a day (in the centre-piece of S. 208, which gives you the types of Hermes). At the top you have an archaic representation of Hermes stealing Io from Argus. Argus is here the Night; his grotesque features monstrous; his hair overshadowing his shoulders; Hermes on tip-toe, stealing upon him and taking the cord which is fastened to the horn of Io out of his hand without his feeling it. Then, underneath, you have the course of an entire day. Apollo first, on the left, dark, entering his chariot, the sun not yet risen. In front of him Artemis, as the moon, ascending before him, playing on her lyre, and looking back to the sun. In the centre, behind the horses, Hermes, as the cumulus cloud at mid-day, wearing his petasus heightened to a cone, and holding a flower in his right hand; indicating the nourishment of the flowers by the rain from the heat cloud. Finally, on the right, Latona, going down as the evening, lighted from the right by the sun, now sunk; and with her feet reverted, signifying the reluctance of the departing day.

Finally, underneath, you have Hermes of the Phidian period, as the floating cumulus cloud, almost shapeless (as you see him at this distance); with the tortoise-shell lyre in his hand, barred with black, and a fleece of white cloud, not level but _oblique_, under his feet. (Compare the "+dia ton koilon--plagiai+," and the relations of the "+aigidos eniochos Athana+," with the clouds as the moon's messengers, in Aristophanes; and note of Hermes generally, that you never find him flying as a Victory flies, but always, if moving fast at all, _clambering_ along, as it were, as a cloud gathers and heaps itself: the Gorgons stretch and stride in their flight, half kneeling, for the same reason, running or gliding shapelessly along in this stealthy way.)

157. And now take this last illustration, of a very different kind. Here is an effect of morning light by Turner (S. 301), on the rocks of Otley-hill, near Leeds, drawn long ago, when Apollo, and Artemis, and Athena, still sometimes were seen, and felt, even near Leeds. The original drawing is one of the great Farnley series, and entirely beautiful. I have shown, in the last volume of "Modern Painters," how well Turner knew the meaning of Greek legends:--he was not thinking of them, however, when he made this design; but, unintentionally, has given us the very effect of morning light we want: the glittering of the sunshine on dewy grass, half dark; and the narrow gleam of it on the sides and head of the stag and hind.

158. These few instances will be enough to show you how we may read in the early art of the Greeks their strong impressions of the power of light. You will find the subject entered into at somewhat greater length in my "Queen of the Air;" and if you will look at the beginning of the 7th book of Plato's "Polity," and read carefully the passages in the context respecting the sun and intellectual sight, you will see how intimately this physical love of light was connected with their philosophy, in its search, as blind and captive, for better knowledge. I shall not attempt to define for you to-day the more complex but much shallower forms which this love of light, and the philosophy that accompanies it, take in the mediaeval mind; only remember that in future, when I briefly speak of the Greek school of art with reference to questions of delineation, I mean the entire range of the schools, from Homer's days to our own, which concern themselves with the representation of light, and the effects it produces on material form--beginning practically for us with these Greek vase paintings, and closing practically for us with Turner's sunset on the Temeraire; being throughout a school of captivity and sadness, but of intense power; and which in its technical method of shadow on material form, as well as in its essential temper, is centrally represented to you by Duerer's two great engravings of the "Melencolia" and the "Knight and Death." On the other hand, when I briefly speak to you of the Gothic school, with reference to delineation, I mean the entire and much more extensive range of schools extending from the earliest art in Central Asia and Egypt down to our own day in India and China:--schools which have been content to obtain beautiful harmonies of colour without any representation of light; and which have, many of them, rested in such imperfect expressions of form as could be so obtained; schools usually in some measure childish, or restricted in intellect, and similarly childish or restricted in their philosophies or faiths: but contented in the restriction; and in the more powerful races, capable of advance to nobler development than the Greek schools, though the consummate art of Europe has only been accomplished by the union of both. How that union was effected, I will endeavour to show you in my next lecture; to-day I shall take note only of the points bearing on our immediate practice.

159. A certain number of you, by faculty and natural disposition,--and all, so far as you are interested in modern art,--will necessarily have to put yourselves under the discipline of the Greek or chiaroscuro school, which is directed primarily to the attainment of the power of representing form by pure contrast of light and shade. I say, the "discipline" of the Greek school, both because, followed faithfully, it is indeed a severe one, and because to follow it at all is, for persons fond of colour, often a course of painful self-denial, from which young students are eager to escape. And yet, when the laws of both schools are rightly obeyed, the most perfect discipline is that of the colourists; for they see and draw _everything_, while the chiaroscurists must leave much indeterminate in mystery, or invisible in gloom: and there are therefore many licentious and vulgar forms of art connected with the chiaroscuro school, both in painting and etching, which have no parallel among the colourists. But both schools, rightly followed, require first of all absolute accuracy of delineation. _This_ you need not hope to escape. Whether you fill your spaces with colours, or with shadows, they must equally be of the true outline and in true gradations. I have been thirty years telling modern students of art this in vain. I mean to say it to you only once, for the statement is too important to be weakened by repetition.


160. It may make this more believable to you if I put beside each other a piece of detail from each school. I gave you the St. John of Cima da Conegliano for a type of the colour school. Here is my own study of the sprays of oak which rise against the sky of it in the distance, enlarged to about its real size (Edu. 12). I hope to draw it better for you at Venice; but this will show you with what perfect care the colourist has followed the outline of every leaf in the sky. Beside, I put a chiaroscurist drawing (at least, a photograph of one), Duerer's from nature, of the common wild wall-cabbage (Edu. 32). It is the most perfect piece of delineation by flat tint I have ever seen, in its mastery of the perspective of every leaf, and its attainment almost of the bloom of texture, merely by its exquisitely tender and decisive laying of the colour. These two examples ought, I think, to satisfy you as to the precision of outline of both schools, and the power of expression which may be obtained by flat tints laid within such outline.

161. Next, here are two examples of the gradated shading expressive of the forms within the outline, by two masters of the chiaroscuro school. The first (S. 12) shows you Lionardo's method of work, both with chalk and the silver point. The second (S. 302), Turner's work in mezzotint; both masters doing their best. Observe that this plate of Turner's, which he worked on so long that it was never published, is of a subject peculiarly depending on effects of mystery and concealment, the fall of the Reuss under the Devil's Bridge on the St. Gothard; (the _old_ bridge; you may still see it under the existing one, which was built since Turner's drawing was made). If ever outline could be dispensed with, you would think it might be so in this confusion of cloud, foam, and darkness. But here is Turner's own etching on the plate (Edu. 35 F), made under the mezzotint; and of all the studies of rock outline made by his hand, it is the most decisive and quietly complete.

162. Again; in the Lionardo sketches, many parts are lost in obscurity, or are left intentionally uncertain and mysterious, even in the light, and you might at first imagine some permission of escape had been here given you from the terrible law of delineation. But the slightest attempts to copy them will show you that the terminal lines are inimitably subtle, unaccusably true, and filled by gradations of shade so determined and measured that the addition of a grain of the lead or chalk as large as the filament of a moth's wing, would make an appreciable difference in them.

This is grievous, you think, and hopeless? No, it is delightful and full of hope: delightful, to see what marvellous things can be done by men; and full of hope, if your hope is the right one, of being one day able to rejoice more in what others have done, than in what you can yourself do, and more in the strength that is for ever above you, than in that you can never attain.

163. But you can attain much, if you will work reverently and patiently, and hope for no success through ill-regulated effort. It is, however, most assuredly at this point of your study that the full strain on your patience will begin. The exercises in line-drawing and flat laying of colour are irksome; but they are definite, and within certain limits, sure to be successful if practised with moderate care. But the expression of form by shadow requires more subtle patience, and involves the necessity of frequent and mortifying failure, not to speak of the self-denial which I said was needful in persons fond of colour, to draw in mere light and shade. If, indeed, you were going to be artists, or could give any great length of time to study, it might be possible for you to learn wholly in the Venetian school, and to reach form through colour. But without the most intense application this is not possible; and practically, it will be necessary for you, as soon as you have gained the power of outlining accurately, and of laying flat colour, to learn to express solid form as shown by light and shade only. And there is this great advantage in doing so, that many forms are more or less disguised by colour, and that we can only represent them completely to others, or rapidly and easily record them for ourselves, by the use of shade alone. A single instance will show you what I mean. Perhaps there are few flowers of which the impression on the eye is more definitely of flat colour, than the scarlet geranium. But you would find, if you were to try to paint it,--first, that no pigment could approach the beauty of its scarlet; and secondly, that the brightness of the hue dazzled the eye, and prevented its following the real arrangement of the cluster of flowers. I have drawn for you here (at least this is a mezzotint from my drawing), a single cluster of the scarlet geranium, in mere light and shade (Edu. 32 B.), and I think you will feel that its domed form, and the flat lying of the petals one over the other, in the vaulted roof of it, can be seen better thus than if they had been painted scarlet.

164. Also this study will be useful to you, in showing how entirely effects of light depend on delineation, and gradation of spaces, and not on methods of shading. And this is the second great practical matter I want you to remember to-day. All effects of light and shade depend not on the method or execution of shadows, but on their rightness of place, form, and depth. There is indeed a loveliness of execution _added_ to the rightness, by the great masters, but you cannot obtain that unless you become one of them. Shadow cannot be laid thoroughly well, any more than lines can be drawn steadily, but by a long-practised hand, and the attempts to imitate the shading of fine draughtsmen, by dotting and hatching, are just as ridiculous as it would be to endeavour to imitate their instantaneous lines by a series of re-touchings. You will often indeed see in Lionardo's work, and in Michael Angelo's, shadow wrought laboriously to an extreme of fineness; but when you look into it, you will find that they have always been drawing more and more form within the space, and never finishing for the sake of added texture, but of added fact. And all those effects of transparency and reflected light, aimed at in common chalk drawings, are wholly spurious. For since, as I told you, all lights are shades compared to higher lights, and lights only as compared to lower ones, it follows that there can be no difference in their quality as such; but that light is opaque when it expresses substance, and transparent when it expresses space; and shade is also opaque when it expresses substance, and transparent when it expresses space. But it is not, even then, transparent in the common sense of that word; nor is its appearance to be obtained by dotting or cross hatching, but by touches so tender as to look like mist. And now we find the use of having Lionardo for our guide. He is supreme in all questions of execution, and in his 28th chapter, you will find that shadows are to be "dolce e sfumose," to be tender, and look as if they were exhaled, or breathed on the paper. Then, look at any of Michael Angelo's finished drawings, or of Correggio's sketches, and you will see that the true nurse of light is in art, as in nature, the cloud; a misty and tender darkness, made lovely by gradation.

165. And how absolutely independent it is of material or method of production, how absolutely dependent on rightness of place and depth,--there are now before you instances enough to prove. Here is Duerer's work in flat colour, represented by the photograph in its smoky brown; Turner's, in washed sepia, and in mezzotint; Lionardo's, in pencil and in chalk; on the screen in front of you a large study in charcoal. In every one of these drawings, the material of shadow is absolutely opaque. But photograph-stain, chalk, lead, ink, or charcoal,--every one of them, laid by the master's hand, becomes full of light by gradation only. Here is a moonlight (Edu. 31 B.), in which you would think the moon shone through every cloud; yet the clouds are mere single dashes of sepia, imitated by the brown stain of a photograph; similarly, in these plates from the Liber Studiorum the white paper becomes transparent or opaque, exactly as the master chooses. Here, on the granite rock of the St. Gothard (S. 302), in white paper made opaque, every light represents solid bosses of rock, or balls of foam. But in this study of twilight (S. 303), the same white paper (coarse old stuff it is, too!) is made as transparent as crystal, and every fragment of it represents clear and far away light in the sky of evening in Italy.

From all which the practical conclusion for you is, that you are never to trouble yourselves with any questions as to the means of shade or light, but only with the right government of the means at your disposal. And it is a most grave error in the system of many of our public drawing-schools, that the students are permitted to spend weeks of labour in giving attractive appearance, by delicacy of texture, to chiaroscuro drawings in which every form is false, and every relation of depth, untrue. A most unhappy form of error; for it not only delays, and often wholly arrests, their advance in their own art; but it prevents what ought to take place correlatively with their executive practice, the formation of their taste by the accurate study of the models from which they draw. And I must so far anticipate what we shall discover when we come to the subject of sculpture, as to tell you the two main principles of good sculpture; first, that its masters think before all other matters of the right placing of masses; secondly, that they give life by flexure of surface, not by quantity of detail; for sculpture is indeed only light and shade drawing in stone.

166. Much that I have endeavoured to teach on this subject has been gravely misunderstood, by both young painters and sculptors, especially by the latter. Because I am always urging them to imitate organic forms, they think if they carve quantities of flowers and leaves, and copy them from the life, they have done all that is needed. But the difficulty is not to carve quantities of leaves. Anybody can do that. The difficulty is, never anywhere to have an _unnecessary_ leaf. Over the arch on the right, you see there is a cluster of seven, with their short stalks springing from a thick stem. Now, you could not turn one of those leaves a hair's-breadth out of its place, nor thicken one of their stems, nor alter the angle at which each slips over the next one, without spoiling the whole as much as you would a piece of melody by missing a note. That is disposition of masses. Again, in the group on the left, while the placing of every leaf is just as skilful, they are made more interesting yet by the lovely undulation of their surfaces, so that not one of them is in equal light with another. And that is so in all good sculpture, without exception. From the Elgin marbles down to the lightest tendril that curls round a capital in the thirteenth century, every piece of stone that has been touched by the hand of a master, becomes soft with under-life, not resembling nature merely in skin-texture, nor in fibres of leaf, or veins of flesh; but in the broad, tender, unspeakably subtle undulation of its organic form.

167. Returning then to the question of our own practice, I believe that all difficulties in method will vanish, if only you cultivate with care enough the habit of accurate observation, and if you think only of making your light and shade true, whether it be delicate or not. But there are three divisions or degrees of truth to be sought for, in light and shade, by three several modes of study, which I must ask you to distinguish carefully.

I. When objects are lighted by the direct rays of the sun, or by direct light entering from a window, one side of them is of course in light, the other in shade, and the forms in the mass are exhibited systematically by the force of the rays falling on it; (those having most power of illumination which strike most vertically;) and note that there is, therefore, to every solid curvature of surface, a necessarily proportioned gradation of light, the gradation on a parabolic solid being different from the gradation on an elliptical or spherical one. Now, when your purpose is to represent and learn the anatomy, or otherwise characteristic forms, of any object, it is best to place it in this kind of direct light, and to draw it as it is seen when we look at it in a direction at right angles to that of the ray. This is the ordinary academical way of studying form. Lionardo seldom practises any other in his real work, though he directs many others in his treatise.

168. The great importance of anatomical knowledge to the painters of the sixteenth century rendered this method of study very frequent with them; it almost wholly regulated their schools of engraving, and has been the most frequent system of drawing in art-schools since (to the very inexpedient exclusion of others). When you study objects in this way,--and it will indeed be well to do so often, though not exclusively,--observe always one main principle. Divide the light from the darkness frankly at first: all over the subject let there be no doubt which is which. Separate them one from the other as they are separated in the moon, or on the world itself, in day and night. Then gradate your lights with the utmost subtilty possible to you; but let your shadows alone, until near the termination of the drawing: then put quickly into them what farther energy they need, thus gaining the reflected lights out of their original flat gloom; but generally not looking much for reflected lights. Nearly all young students (and too many advanced masters) exaggerate them. It is good to see a drawing come out of its ground like a vision of light only; the shadows lost, or disregarded in the vague of space. In vulgar chiaroscuro the shades are so full of reflection that they look as if some one had been walking round the object with a candle, and the student, by that help, peering into its crannies.

169. II. But, in the reality of nature, very few objects are seen in this accurately lateral manner, or lighted by unconfused direct rays. Some are all in shadow, some all in light, some near, and vigorously defined; others dim and faint in aerial distance. The study of these various effects and forces of light, which we may call aerial chiaroscuro, is a far more subtle one than that of the rays exhibiting organic form (which for distinction's sake we may call "formal" chiaroscuro), since the degrees of light from the sun itself to the blackness of night, are far beyond any literal imitation. In order to produce a mental impression of the facts, two distinct methods may be followed:--the first, to shade downwards from the lights, making everything darker in due proportion, until the scale of our power being ended, the mass of the picture is lost in shade. The second, to assume the points of extreme darkness for a basis, and to light everything above these in due proportion, till the mass of the picture is lost in light.

170. Thus, in Turner's sepia drawing "Isis" (Edu. 31), he begins with the extreme light in the sky, and shades down from that till he is forced to represent the near trees and pool as one mass of blackness. In his drawing of the Greta (S. 2), he begins with the dark brown shadow of the bank on the left, and illuminates up from that, till, in his distance, trees, hills, sky, and clouds, are all lost in broad light, so that you can hardly see the distinction between hills and sky. The second of these methods is in general the best for colour, though great painters unite both in their practice, according to the character of their subject. The first method is never pursued in colour but by inferior painters. It is, nevertheless, of great importance to make studies of chiaroscuro in this first manner for some time, as a preparation for colouring; and this for many reasons, which it would take too long to state now. I shall expect you to have confidence in me when I assure you of the necessity of this study, and ask you to make good use of the examples from the Liber Studiorum which I have placed in your Educational series.

171. III. Whether in formal or aerial chiaroscuro, it is optional with the student to make the local colour of objects a part of his shadow, or to consider the high lights of every colour as white. For instance, a chiaroscurist of Lionardo's school, drawing a leopard, would take no notice whatever of the spots, but only give the shadows which expressed the anatomy. And it is indeed necessary to be able to do this, and to make drawings of the forms of things as if they were sculptured, and had no colour. But in general, and more especially in the practice which is to guide you to colour, it is better to regard the local colour as part of the general dark and light to be imitated; and, as I told you at first, to consider all nature merely as a mosaic of different colours, to be imitated one by one in simplicity. But good artists vary their methods according to their subject and material. In general, Duerer takes little account of local colour; but in woodcuts of armorial bearings (one with peacock's feathers I shall get for you some day) takes great delight in it; while one of the chief merits of Bewick is the ease and vigour with which he uses his black and white for the colours of plumes. Also, every great artist looks for, and expresses, that character of his subject which is best to be rendered by the instrument in his hand, and the material he works on. Give Velasquez or Veronese a leopard to paint, the first thing they think of will be its spots; give it to Duerer to engrave, and he will set himself at the fur and whiskers; give it a Greek to carve, and he will only think of its jaws and limbs; each doing what is absolutely best with the means at his disposal.

172. The details of practice in these various methods I will endeavour to explain to you by distinct examples in your Educational series, as we proceed in our work; for the present, let me, in closing, recommend to you once more with great earnestness the patient endeavour to render the chiaroscuro of landscape in the manner of the Liber Studiorum; and this the rather, because you might easily suppose that the facility of obtaining photographs which render such effects, as it seems, with absolute truth and with unapproachable subtilty, superseded the necessity of study, and the use of sketching. Let me assure you, once for all, that photographs supersede no single quality nor use of fine art, and have so much in common with Nature, that they even share her temper of parsimony, and will themselves give you nothing valuable that you do not work for. They supersede no good art, for the definition of art is "human labour regulated by human design," and this design, or evidence of active intellect in choice and arrangement, is the essential part of the work; which so long as you cannot perceive, you perceive no art whatsoever; which when once you do perceive, you will perceive also to be replaceable by no mechanism. But, farther, photographs will give you nothing you do not work for. They are invaluable for record of some kinds of facts, and for giving transcripts of drawings by great masters; but neither in the photographed scene, nor photographed drawing, will you see any true good, more than in the things themselves, until you have given the appointed price in your own attention and toil. And when once you have paid this price, you will not care for photographs of landscape. They are not true, though they seem so. They are merely spoiled nature. If it is not human design you are looking for, there is more beauty in the next wayside bank than in all the sun-blackened paper you could collect in a lifetime. Go and look at the real landscape, and take care of it; do not think you can get the good of it in a black stain portable in a folio. But if you care for human thought and passion, then learn yourselves to watch the course and fall of the light by whose influence you live, and to share in the joy of human spirits in the heavenly gifts of sunbeam and shade. For I tell you truly, that to a quiet heart, and healthy brain, and industrious hand, there is more delight, and use, in the dappling of one wood-glade with flowers and sunshine, than to the restless, heartless, and idle could be brought by a panorama of a belt of the world, photographed round the equator.

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1911 encyclopedia

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From LoveToKnow 1911

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Up to date as of January 15, 2010
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Definition from Wiktionary, a free dictionary

Wikipedia has articles on:




Most common English words: Gutenberg « best « word « #247: light » felt » since » use


Etymology 1

Old English līhtan (illuminate)


to light

Third person singular

Simple past
lit or lighted

Past participle
[[lit or lighted]]

Present participle

to light (third-person singular simple present lights, present participle lighting, simple past and past participle lit or lighted)

  1. (transitive) To start (a fire).
  2. (transitive) To illuminate.
Derived terms
The translations below need to be checked and inserted above into the appropriate translation tables, removing any numbers. Numbers do not necessarily match those in definitions. See instructions at Help:How to check translations.

Etymology 2

Old English lēoht. Cognate with Dutch licht, German Licht.


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light (plural lights)

  1. (uncountable) The natural medium emanating from the sun and other very hot sources (now recognised as electromagnetic radiation with a wavelength of 400-750 nm), within which vision is possible.
    As you can see, this spacious dining-room gets a lot of light in the mornings.
  2. A source of illumination.
    Put that light out!
  3. Spiritual or mental illumination; enlightenment, useful information.
    Can you throw any light on this problem?
  4. (in plural, now rare) Facts. pieces of information; ideas, concepts.
    • 1621, Robert Burton, The Anatomy of Melancholy, Book I, New York 2001, p. 166:
      Now these notions are twofold, actions or habits [...], which are durable lights and notions, which we may use when we will.
  5. A notable person within a specific field or discipline.
    Picasso was one of the leading lights of the cubist movement.
  6. A point of view, or aspect from which a concept, person or thing is regarded.
    I'm really seeing you in a different light today.
    Magoon's governorship in Cuba was viewed in a negative light by many Cuban historians for years thereafter.
  7. A flame or something used to create fire.
    Hey, buddy, you got a light?
  8. A window, or space for a window in architecture
    This facade has eight south-facing lights.
  9. The series of squares reserved for the answer to a crossword clue
    The average length of a light on a 15x15 grid is 7 or 8.
  10. (informal) A cross-light in a double acrostic or triple acrostic.
Derived terms
The translations below need to be checked and inserted above into the appropriate translation tables, removing any numbers. Numbers do not necessarily match those in definitions. See instructions at Help:How to check translations.


light (comparative lighter, superlative lightest)

  1. having light
  2. pale in colour
  3. (of coffee) served with extra milk or cream
Derived terms
  • light-haired
  • light-skinned
The translations below need to be checked and inserted above into the appropriate translation tables, removing any numbers. Numbers do not necessarily match those in definitions. See instructions at Help:How to check translations.

Etymology 3

Old English lēocht. Cognate with Dutch licht, German leicht, Swedish lätt, Norwegian lett.


light (comparative lighter, superlative lightest)

  1. Of low weight; not heavy.
    My bag was much lighter once I had dropped off the books.
  2. Lightly-built; designed for speed or small loads.
    We took a light aircraft down to the city.
  3. Gentle; having little force or momentum.
    This artist clearly had a light, flowing touch.
  4. Low in fat, calories, alcohol, salt, etc.
    This light beer still gets you drunk if you have enough of it.
  5. Unimportant, trivial, having little value or significance.
    I made some light comment, and we moved on.
Derived terms
The translations below need to be checked and inserted above into the appropriate translation tables, removing any numbers. Numbers do not necessarily match those in definitions. See instructions at Help:How to check translations.


light (comparative lighter, superlative lightest)

  1. Carrying little.
    I prefer to travel light.




light (plural lights)

  1. (curling) A stone that is not thrown hard enough.


to light

Third person singular

Simple past

Past participle

Present participle

to light (third-person singular simple present lights, present participle lighting, simple past and past participle lighted)

  1. (nautical) To unload a ship, or to jettison material to make it lighter
Derived terms

Etymology 4

Old English līhtan


to light

Third person singular

Simple past
lit or lighted

Past participle
[[lit or lighted]]

Present participle

to light (third-person singular simple present lights, present participle lighting, simple past and past participle lit or lighted)

  1. To find by chance.
    I lit upon a rare book in a second-hand bookseller's.
  2. (archaic) To alight.
    She fell out of the window but luckily lit on her feet.
Derived terms


Up to date as of January 23, 2010
(Redirected to Physics Course/Light article)

From Wikibooks, the open-content textbooks collection

< Physics Course

Sources of Light

Light generates from difference sources .

  • Light from Celestial Bodies. Example(s): Light from Stars, Radiation from Black Holes.
  • Light from Fire . A chemical reaction between flammable and combustible materials . Example(s): a Candle's Light ,a Matches' Light.
  • Radiated Electromagnetic Lights . Matter Absorbs Electric Energy until one point in time it no longer absorb Electric Energy instead it converts Electric Energy into Radiated Electromagnetic Light.
  • Light from Life. Bioluminesence. Example(s): Fireflies, Certain Bacteria.
  • Light from Electric Discharge . Light from thunder

Characteristics of Light

  • Light travels as a wave at a constant speed C = 3 x 1018 m/s
  • Light shines on an object will leave objects' image
  • The absorption of Light Energy depend on matter material's Thickness and Color .
Thick Clothes are easy to dry under the sun light . Dark Clothes are easy to dry under the sun light
  • Light Wave from different sources travel in the same medium can interfere with each other to produce different kinds of Light's Interference . No Light, when light waves interfere constructively . Bright Light when light waves interfere Destructively
  • Light Wave can be reflected back into the medium that it comes from and could be interfere with other wave lights in the medium to produce Interference
  • Light Decomposed into Six lights of six colors Red , Orange , Yellow , Green , Blue , Violet when it travels through a crystal medium . For example Rainbow in the sky after the rain . Light passing through Prism
  • Color of light can be produced from three primaries colors namely Red Green Blue commonly refer to as RGB by mixing them in percentage
  • Light that the eyes perceive are in the frequency length of that has wavelength 400 - 700 nm

Light and Matter

Bible wiki

Up to date as of January 23, 2010

From BibleWiki

the offspring of the divine command (Gen 1:3). "All the more joyous emotions of the mind, all the pleasing sensations of the frame, all the happy hours of domestic intercourse were habitually described among the Hebrews under imagery derived from light" (1 Kg 11:36; Isa 58:8; Est 8:16; Ps 9711). Light came also naturally to typify true religion and the felicity it imparts (Ps 119105; Isa 8:20; Mt 4:16, etc.), and the glorious inheritance of the redeemed (Col 1:12; Rev 21:23-25). God is said to dwell in light inaccessible (1 Tim 6:16). It frequently signifies instruction (Mt 5:16; Jn 5:35). In its highest sense it is applied to Christ as the "Sun of righteousness" (Mal 4:2; Lk 2:32; Jn 1:7-9). God is styled "the Father of lights" (James 1:17). It is used of angels (2Cor 11:14), and of John the Baptist, who was a "burning and a shining light" (Jn 5:35), and of all true disciples, who are styled "the light of the world" (Mt 5:14).

This entry includes text from Easton's Bible Dictionary, 1897.

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This article needs to be merged with LIGHT (Jewish Encyclopedia).

Simple English

Light is a type of energy. It is a form of electromagnetic radiation of a wavelength which can be detected by the human eye.[1] In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.[2][3] Light exists in tiny packets called photons. It shows properties of both waves and particles. The study of light, known as optics, is an important research area in modern physics.

Five main properties of light are intensity, frequency or wavelength, polarization, phase and orbital angular momentum.

About light

In a vacuum (where there is nothing; so, when not slowed down by other particles), light moves at the speed of light; which is 299,792,458 meters, or about 186,282 miles, per second. This means it takes about 8 minutes for light to reach Earth from the sun.[4][5] In glass it travels at about two-thirds this fast.

Light moves in a straight line, creating shadows when the path of light is blocked. More solid things will have a darker shadow, things that are more clear have a lighter shadow, and transparent things will have none or very little shadow. Light can pass through transparent things the most easily. Our eyes react to light; when we see something we see the light it reflects, or the light it emits. For example, a lamp gives off light, and everything else in the same room as the lamp reflects its light.

Every colour of light has a different wavelength. The shorter the wavelength, the more energy the light has. The speed at which light moves does not depend on its energy. Going through partly clear objects can slow light down by a very small amount.

White light is made up of many different colors of light added together. When white light shines through a prism, it splits up into different colors, becoming a spectrum. The spectrum contains all of the wavelengths of light that we can see. Red light has the longest wavelength, and violet (purple) light has the shortest.

Light with a wavelength shorter than violet is called ultraviolet light. X-rays and Gamma rays are also forms of light with even shorter wavelengths than ultraviolet. Light with a wavelength longer than red is called infrared light. Radio waves are a form of electromagnetic radiation with a wavelength even longer than infrared light. The microwaves that are used to heat food in a microwave oven are also a form of electromagnetic radiation . Our eyes cannot see those kinds of energy, but there are some cameras that can see them. The various forms of light, both visible and invisible are the electromagnetic spectrum.

When light is refracted in raindrops, a rainbow is made. The raindrop acts like a prism and refracts the light until we can see the colors of the spectrum.


  1. International Commission on Illumination 1987. International Lighting Vocabulary. Number 17.4. CIE, 4th edition. ISBN 978-3-900734-07-7.
    By the International Lighting Vocabulary, the definition of light is: “Any radiation capable of causing a visual sensation directly.”
  2. Gregory Hallock Smith (2006), Camera lenses: from box camera to digital, SPIE Press, p. 4, ISBN 9780819460936, 
  3. Narinder Kumar (2008), Comprehensive Physics XII, Laxmi Publications, p. 1416, ISBN 9788170085928, 
  4. "Seeing in the Dark . Astronomy Topics . Light as a Cosmic Time Machine". Retrieved 13 August 2010. 
  5. "Cosmic Distance Scales - The Solar System". Retrieved 13 August 2010. 

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