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For other uses, see Mirage (disambiguation)
The upper frame is an inferior mirage of the Farallon Islands. The second frame is the Farallon Islands with a green flash on the left-hand side. The two lower frames and the main frame are superior mirages of the Farallon Islands. The superior mirage went from a 3-image mirage (inverted image between erect ones) to a 5-image mirage to 2-image mirage. Such a display is consistent with a Fata Morgana. All frames but the upper one were photographed from about 50–70 feet above sea level. The upper frame was photographed from sea level. The interval between the first and last frames of the superior mirage was six minutes.

A mirage is a naturally occurring optical phenomenon in which light rays are bent to produce a displaced image of distant objects or the sky. The word comes to English via the French mirage, from the Latin mirare, meaning "to look at, to wonder at". This is the same root as for "mirror" and "to admire".

Like a mirror, a mirage shows images of things which are elsewhere. The principal physical cause of a mirage, however, is refraction rather than reflection.

In contrast to a hallucination, a mirage is a real optical phenomenon which can be captured on camera, since light rays actually are refracted to form the false image at the observer's location. What the image appears to represent, however, is determined by the interpretive faculties of the human mind. For example, inferior images on land are very easily mistaken for the reflections from a small body of water.

Mirages can be categorized as "inferior" (meaning lower), "superior" (meaning higher) and "Fata Morgana", a kind of superior mirage which consist of a series of unusually elaborate, vertically-stacked images which form one rapidly-changing mirage.

Contents

Cause

The cause of an inferior mirage (light curvature greatly exaggerated)

Cold air is denser than warm air and has therefore a greater refractive index. As light passes from colder air across a sharp boundary to significantly warmer air, the light rays bend away from the direction of the temperature gradient. When light rays pass from hotter to colder, they bend toward the direction of the gradient. If the air near the ground is warmer than that higher up, the light ray bends in a concave, upward trajectory.

Once the ray reaches the viewer’s eye, the visual cortex interprets it as if it traces back along a perfectly straight "line of sight". This line is however at a tangent to the path the ray takes at the point it reaches the eye. The result is that an "inferior image" of the sky above appears on the ground. The viewer may incorrectly interpret this sight as water which is reflecting the sky, which is, to the brain, a more reasonable and common occurrence.

In the case where the air near the ground is cooler than that higher up, the light rays curve downward, producing a "superior image".

The "resting" state of the Earth's atmosphere has a vertical gradient of about -1° Celsius per 100 metres of altitude. (The value is negative because it gets colder as altitude increases.) For a mirage to happen, the temperature gradient has to be much greater than that. According to Minnaert,[1] the magnitude of the gradient needs to be at least 2°C per meter, and the mirage does not get strong until the magnitude reaches 4º or 5°C per meter. These conditions do occur when there is strong heating at ground level, for example when the sun has been shining on sand or asphalt and an inferior image is commonly generated because of this.

Inferior mirage

An inferior mirage on the Mojave Desert in spring

The model given above explains the cause of the inferior mirage, called "inferior" because the image seen is under the real object. The real object is the (blue) sky or any distant object in that direction, meaning we see a bright bluish patch on the ground in the distance. For exhausted travelers in the desert it appears as a lake of water. On tarmac roads it may seem that water or even oil has been spilled. This is called a "desert mirage" or "highway mirage". Note that both sand and tarmac can become very hot when exposed to the sun, easily being more than 10°C hotter than the air one meter above, enough to cause the mirage.

Light rays coming from a particular distant object all travel through nearly the same air layers and all are bent over about the same amount. Therefore rays coming from the top of the object will arrive lower than those from the bottom. The image usually is upside down, enhancing the illusion that the sky image seen in the distance is really a water or oil puddle acting as a mirror.

Inferior images are not stable. Hot air rises, and cooler air (being more dense) descends, so the layers will mix, giving rise to turbulence. The image will be distorted accordingly. It may be vibrating; it may be vertically extended (towering) or horizontally extended (stooping). If there are several temperature layers, several mirages may mix together, perhaps causing double images. In any case, mirages are usually not larger than about half a degree high (same apparent size as the sun and moon) and from objects only a few kilometers away.

Highway mirage

A hot road mirage, "fake water" on the road, the most common example of an inferior mirage

A highway mirage is an inferior mirage which can be seen very commonly on roadways by day or by night. The hot-road mirage is usually most noticeable on hot sunny days.

Warm air is less dense than cool air, and the variation between the hot air at the surface of the road and the denser cool air above it creates a gradient in the refractive index of the air. Light from the sky at a shallow angle to the road is refracted by the index gradient, making it appear as if the sky is reflected by the road's surface. The result looks to the human mind like a pool of water on the road, since water also reflects the sky.

Superior mirage

Superior mirage of a distant land

A superior mirage occurs when the air below the line of sight is colder than that above. This is called a temperature inversion, since it does not represent the normal equilibrium temperature gradient of the atmosphere. Since in this case the light rays are bent down, the image appears above the true object, hence the name superior. They are in general less common than inferior mirages, but when they do occur they tend to be more stable, as cold air has no tendency to move up or warm air to move down.

Superior mirages are most common in polar regions, especially over large sheets of ice with a uniform low temperature. They also occur at more moderate latitudes, however, although in that case they are weaker and not so smooth. For example a distant shoreline may be made towering, looking higher (and thus perhaps closer) than it is in reality, but because of the turbulences there seem to be dancing spikes, towers and so forth. This type of mirage is also called the Fata Morgana or, in Icelandic, halgerndingar.

Superior images can be right-side-up or upside down, depending on the distance of the true object and the temperature gradient. Often the image appears as a distorted mixture of up and down parts.

Superior mirages are able to have such a striking effect due to the Earth's curvature. Were the Earth flat, light rays which bent down would soon hit the ground and only close objects would be affected. As the Earth is round, if the amount of downward bending is about equal to the curvature of the Earth, light rays can travel large distances, perhaps from beyond the horizon. This was observed for the first time in 1596, when a ship under the command of Willem Barents looking for the Northeast passage got stuck in the ice at Novaya Zemlya, and the crew had to endure the polar winter there. They saw their midwinter night ending with the rise of a distorted sun about 2 weeks earlier than expected. It was not until the 20th century that Europeans understood the reason: the real sun had still been under their horizon, but its light rays followed the curvature of the Earth. This effect is often called a Novaya Zemlya mirage. For every 100 kilometres (62 mi) the light rays can travel parallel to the Earth's surface, the sun will appear 1° higher on the horizon. The inversion layer must have just the right temperature gradient over the whole distance to make this possible.

In the same way, ships which are in reality so far away that they should not be visible above the geometric horizon may appear on the horizon, or even above the horizon, as superior mirages. This may explain some stories about flying ships or coastal cities in the sky, as described by some polar explorers. These are examples of so-called Arctic mirages, or hillingar in Icelandic.

If the vertical temperature gradient is +11°C per 100 meters (reminder: positive means getting hotter when going up), horizontal light rays will just follow the curvature of the Earth, and the horizon will appear flat. If the gradient is less the rays are not bent enough and get lost in space. That is the normal situation of a spherical, convex horizon. But if the gradient gets larger, say 18°C per 100 meters, the observer will see the horizon turned upwards, being concave, as if he were standing at the bottom of a saucer.

Fata Morgana

Sequence of a Fata Morgana of the Farallon Islands as seen from San Francisco

A Fata Morgana, the name of which comes from the Italian translation of Morgan le Fay, the fairy shapeshifting half-sister of King Arthur, is a very complex superior mirage, with alternations of compressed and stretched zones, and erect and inverted images.[2]Fata Morgana is also a fast changing mirage.

Fata Morgana is most common in polar regions, especially over large sheets of ice with a uniform low temperature, but it can be observed almost anywhere. While in polar regions Fata Morgana is observed on cold days, in deserts and over oceans and lakes Fata Morgana is observed on hot days. For a Fata Morgana, temperature inversion has to be strong enough that light rays' curvatures within the inversion are stronger than the curvature of the Earth[2].

The rays will bend and create arcs. An observer needs to be within a duct in order to be able to see a Fata Morgana.[3] Fata Morgana may be observed from any altitude within the Earth's atmosphere, including from mountaintops or airplanes.[4][5]

A Fata Morgana can go from superior to inferior mirage and back within a few seconds, depending on the constantly changing conditions of the atmosphere. Sixteen frames of the mirage of the Farallon Islands, which cannot be seen from sea level at all under normal conditions because they are located below the horizon, were photographed on the same day. The first fourteen frames have elements of a Fata Morgana display—alternations of compressed and stretched zones.[2] The last two frames were photographed a few hours later around sunset. The air was cooler while the ocean was probably a little bit warmer, which made temperature inversion lower. The mirage was still present, but it was not as complex as it had been a few hours before sunset, and it corresponded no longer to a Fata Morgana but rather to a superior mirage display.

Distortions of image and bending of light can produce spectacular effects. In his book Pursuit: The Chase and Sinking of the "Bismarck," the author Ludovic Kennedy describes an incident that allegedly took place below the Denmark Strait during 1941, following the sinking of the "Hood." The Bismark, while pursued by the British cruisers Norfolk and Suffolk, passed out of sight into a sea mist. Within a matter of seconds, the ship re-appeared steaming toward the British ships at high speed. In alarm the cruisers separated, anticipating an imminent attack, and observers from both ships watched in astonishment as the German Battleship fluttered, grew indistinct and faded away. Radar watch during these events indicated that the Bismark had in fact made no changes of course.

Mirage of astronomical objects

A mirage of an astronomical object is a naturally-occurring optical phenomenon, in which light rays are bent to produce distorted or multiple images of an astronomical object. The mirages might be observed for such astronomical objects as the Sun, the Moon, the planets, bright stars and very bright comets . The most commonly observed are sunset and sunrise mirages.

References

  1. ^ M. Minnaert; The Nature of Light and Colour in the Open Air; 1948; ISBN 0-486-20196-1.
  2. ^ a b c An Introduction to Mirages by Andy Young
  3. ^ [1] by Andy Young
  4. ^ Durst and Bull (1956). Met. Mag. 85. pp. 237–242. 
  5. ^ Andrew, Young. Annotated bibliography of mirages, green flashes, atmospheric refraction, etc.. http://mintaka.sdsu.edu/GF/bibliog/bibliog.html. 

See also

External links


Source material

Up to date as of January 22, 2010

From Wikisource

Mirage
by Clark Ashton Smith
1922.

Deem ye the veiling vision will abide—
The marvel, and the glamor, and the dream
Which lies in light upon the barren world ? . . .

The wings of Phoenix towering to the sun,
Nor opals, nor the morning foam, may hold
The hueful light that as from faery moons
Is mirrored on the sand; where many a time,
From fields that hem with golden asphodel
A river like a dragon coiled in light,
Rise to the noon the hovering minarets
And soaring walls of cities Ilion-like,
Till the dim winds are hung with palaces
Of orient madreperl.

For ever lost—
Like sunset on a land of old romance—
The splendor fails, and leaves the traveller
In bournless deserts flaunting to the day.

PD-icon.svg This work is in the public domain in the United States because it was published before January 1, 1923. It may be copyrighted outside the U.S. (see Help:Public domain). Flag of the United States.svg

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

MIRAGE (a French word, from mirer, to look at, se mirer, to be reflected), an optical illusion due to variations in the refractive index of the atmosphere. It embraces the phenomena of the visionary appearance of lakes in arid deserts, the images of ships and icebergs, frequently seen as if inverted and suspended in the atmosphere in the Polar Regions, the Fata Morgana, and "looming" as witnessed in mists or fogs.

In the article Refraction it is shown that a ray of light traversing a homogeneous medium is deviated from its rectilinear path when it enters a medium of different refractive index; it is therefore readily seen that the path of a ray through continuously varying media is necessarily curvilinear, being compounded of an infinite number of infinitesimally small rectilinear deviations. Our atmosphere is a medium of continuously varying refractive index. Meteorological optical phenomena, due to variations in the refractive index of the atmosphere, may be divided into groups: (I) those due to the permanent or normal variation experienced as one ascends in the atmosphere, and (2) those due to sporadic variations occasioned by irregular heating. The first variation must be taken into account in correcting geodetic observations of heights and astronomical observations of the heavenly bodies; it also has a considerable bearing on the phenomena of the twilight and the afterglow (see Refraction: § Astronomical; and Twilight). The second (or temperature) variation gives rise to phenomena which we proceed to discuss.

A common type of mirage is the appearance of an isolated lake frequently seen in hot sandy deserts, as in the Sahara, Turkestan, &c. The explanation is as follows: The sand, being abnormally heated by the solar rays, causes the neighbouring air to expand, consequently its density, and therefore its refractive index, is diminished, and attains a minimum value in the lowest layers. It increases as we ascend and reaches a maximum at a certain height, and then decreases according to the normal variation. Any object viewed across such an area is seen by two sets of rays: one set passing near the earth and assuming a curved path convex to the horizon, the second set more remote from the earth and concave to the horizon. The object thus appears double, an image being seen mirrored in the sand. The sky appears as a shining lake; mountains or palms may be similarly reflected, but it is to be noted that the images are inverted (see fig.).

Similar atmospheric condition sometimes prevail in the air over large bodies of water on cold autumn mornings. These phenomena have been experimentally realized by R. W. Wood (Phil. Meg., 1899, vol. xlvii.), who viewed objects over a series of heated slate slabs.

Another type of mirage, frequently observed at sea in the northern latitudes, is presented in the appearance of ships and icebergs as if inverted and suspended in the clouds. This is due to a stratum of hot air at some distance above the sea level, the rays of light near the horizon being practically horizontal, while those at greater elevations are fairly concave. It may happen that the change in density is so great that only the upper rays reach the eye; we are then met with the curious illusion of seeing inverted ships in the clouds, although nothing is visible on the ocean.

The Fata Morgana, frequently seen in the Straits of Messina, consists of an apparent vertical elongation of an object situated on the opposite shore. The distribution of density is similar to that attending a desert mirage, but the transition is not so abrupt. The object is really viewed through a horizontally stratified medium consisting of a central sheet of maximum refractive index, overand under-laid by sheets of decreasing refractive power. The system consequently acts as a continuous lens, magnifying the object in a vertical direction.

If, in addition to this horizontal stratification, the atmosphere varies similarly in vertical planes, then the object would be magnified both horizontally and vertically. These conditions sometimes prevail in misty or foggy weather, more particularly at sea, and thus give rise to the phenomena known as "looming." A famous example is the Brockengespenst or "spectre of the Brocken." The chromatic halos which frequently encircle these images are due to diffraction. (See Corona.) It is interesting to note that lenses formed on non-homogeneous. material, having the maximum refractive index along the central axis, have been prepared, and reproduce the effects caused by abnormal distribution of the density of the atmosphere.

The mathematical investigation of this subject was worked out by Gaspard Monge. For this aspect and further details, both descriptive and experimental, see J. Pernter, Meteorologische Optik (1906); E. Mascart, Traite d'optique (1899-1903); R. W. Wood, Physical Optics (1905); R. S. Heath, Geometrical Optics.


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