View source for Mirage

{{Short description|Optical illusion caused by bending of light}}
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{{Refimprove|date=January 2020}}
[[File:Farallon Islands at inferior mirage no mirage and superior mirage.jpg|thumb|upright=1.5|Various kinds of mirages in one location taken over the course of six minutes, not shown in chronological order.{{efn|The uppermost inset frame shows an inferior mirage of the [[Farallon Islands]]. The second inset frame is the Farallon Islands with a [[green flash]] on the left-hand side. The two lower frames and the main frame all show superior mirages of the Farallon Islands. In these three frames, the superior mirages evolve from a 3-image mirage (an inverted image between two erect ones) to a 5-image mirage, and then back to a 2-image mirage. Such a display is consistent with a [[Fata Morgana (mirage)|Fata Morgana]]. All frames but the upper one were photographed about {{convert|50|-|70|ft}} above sea level. The upper frame was photographed from [[sea level]].}}]] A '''mirage''' is a naturally occurring [[Optical phenomena|optical phenomenon]] in which light rays bend via [[refraction]] to produce a displaced image of distant objects or the sky.{{sfn|Lynch|Livingston|Livingston|2001}} The word comes to [[English language|English]] via the [[French language|French]] ''(se) mirer'', from the [[Latin]] ''mirari'', meaning "to look at, to wonder at".<ref>{{Cite Merriam-Webster | Mirage | access-date=2019-12-08 }}</ref>

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

In contrast to a [[hallucination]], a mirage is a real optical phenomenon that can be captured on camera, since light rays are actually 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.

==Inferior mirage==
[[File:Mirage-diagram.svg|thumb|350px|A schematic of an inferior mirage, showing a) the unrefracted [[line of sight]], b) the refracted line of sight and c) the apparent position of the refracted image.]]
In an inferior mirage, the mirage image appears below the real object. The real object in an inferior mirage is the (blue) [[sky]] or any distant (therefore bluish) object in that same direction. The mirage causes the observer to see a bright and bluish patch on the ground.

[[ray (optics)|Light rays]] coming from a particular distant object all travel through nearly the same layers of air, and all are [[refraction|refracted]] at about the same [[Snell's law|angle]]. Therefore, rays coming from the top of the object will arrive lower than those from the bottom. The image is usually upside-down, enhancing the [[illusion]] that the sky image seen in the distance is a [[specular reflection]] on a puddle of water or oil acting as a [[mirror]].

While the aero-dynamics are highly active, the image of the inferior mirage is stable, unlike the fata morgana, which can change within seconds. Since warmer air rises while cooler air (being [[density|denser]]) sinks, the layers will mix, causing [[turbulence]]. The image will be [[distortion|distorted]] accordingly; it may vibrate or be stretched vertically ([[Looming and similar refraction phenomena#Towering and stooping|towering]]) or compressed vertically ([[Looming and similar refraction phenomena#Stooping|stooping]]). A combination of vibration and extension are also possible. If several [[planetary boundary layer|temperature layers]] are present, several mirages may mix, perhaps causing double images. In any case, mirages are usually not larger than about half a degree high (roughly the [[angular diameter]] of the Sun and Moon) and are from objects between dozens of meters and a few kilometers away.

<gallery widths="200px" heights="140px">
File:Desertmirage.jpg|An inferior mirage seen in the [[Mojave Desert]] in a Nevada [[spring (season)|spring]]
File:Inferior Mirage Grand Traverse Bay.jpg|An inferior mirage over [[Grand Traverse Bay]] in [[Michigan]]
</gallery>
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===Heat haze===
{{see also|Schlieren|Twinkling|Astronomical seeing}}
{{Multiple image
 | direction   = horizontal
 | total_width = 500
 | image1      = Hot road mirage.jpg
 | caption1    = A hot-road mirage, in which "fake water" appears on the road, is the most commonly observed instance of an inferior mirage.
 | image2      = Swiss F-5E at Fairford.JPG
 | caption2    = Heat haze seen through [[exhaust gas]] from a jet engine
}} 

''Heat haze'', also called ''heat shimmer'', refers to the inferior mirage observed when viewing objects through a [[air mass (astronomy)|mass]] of heated air. Common instances when heat haze occurs include images of objects viewed across [[asphalt concrete]] (also known as [[Tarmacadam|tarmac]]), roads, and over masonry rooftops on hot days, above and behind fire (as in burning [[candle]]s, [[patio heater]]s, and [[campfire]]s), and through [[exhaust gas]]es from [[jet engine]]s. When appearing on roads due to the hot asphalt, it is often referred to as a "highway mirage". It also occurs in deserts; in that case, it is referred to as a "desert mirage". Both tarmac and sand can become very hot when exposed to the sun, easily being more than {{convert|10|C-change}} higher than the air {{one2a|{{convert|1|m|ft|abbr=off|spell=in|sp=us}}}} above, enough to make conditions suitable to cause the mirage.

[[Convection]] causes the temperature of the air to vary, and the variation between the hot air at the surface of the road and the denser cool air above it causes a [[gradient index optics|gradient]] in the [[refractive index]] of the air. This produces a blurred [[schlieren|shimmering effect]], which hinders the ability to [[optical resolution|resolve]] the image and increases when the image is [[magnification|magnified]] through a [[telescope]] or [[telephoto lens]].

Light from the sky at a shallow angle to the road is [[refraction|refracted]] by the index gradient, making it appear as if the sky is reflected by the road's surface. This might appear as a pool of liquid (usually water, but possibly others, such as oil) on the road, as some types of liquid also reflect the sky. The illusion moves into the distance as the observer approaches the miraged object giving one the same effect as approaching a rainbow.

Heat haze is not related to the atmospheric phenomenon of [[haze]].
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==Superior mirage==
[[File:Mirage_principle.svg|thumb|left|Comparison of inferior and superior mirages due to differing air refractive indices ''n'']]

{{Multiple image
 | direction = horizontal 
 | width     = 330
 | image_gap = 10
 | image1    = MirageAntarctique.jpg
 | caption1  = Above: A superior mirage of a plane on ice, [[McMurdo Station]]<br><br />Right: An artificial mirage, using sugar solutions to simulate the inversion layers.{{efn|A cat is seen looking through a glass, which has three layers of solution, with decreasing [[refractive index]] from bottom to top. The cat appears in multiple images. This simulates an atmosphere with two inversion layers.}}
 | image2    = Cat Mirage.jpg}}

A superior mirage is one in which the mirage image appears to be located above the real object.  A superior mirage occurs when the air below the line of sight is colder than the air above it. This unusual arrangement is called a [[temperature inversion]]. During the daytime, the normal temperature gradient of the atmosphere is cold air above warm air. Passing through the temperature inversion, the light rays are bent down, and so the image appears above the true object, hence the name ''superior''.<ref>{{cite news|url=https://www.bbc.co.uk/news/uk-england-cornwall-56286719|title='Hovering ship' photographed off Cornish coast by walker|work=BBC News |date=4 March 2021}}</ref> 

Superior mirages are quite common in [[polar region]]s, especially over large sheets of ice that have a uniform low temperature. Superior mirages also occur at more moderate latitudes; however, in those cases, they are weaker and tend to be less smooth and stable. For example, a distant shoreline may appear to ''tower'' and look higher (and, thus, perhaps closer) than it really is. Because of the turbulence, there appear to be dancing spikes and towers. This type of mirage is also called the [[Fata Morgana (mirage)|Fata Morgana]], or ''hafgerðingar'' in the [[Icelandic language]].{{sfn|Lynch|Livingston|Livingston|2001|p=58}}

A superior mirage 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.

Since the earth is round, if the downward bending curvature of light rays is about the same as the [[Curvature|curvature of Earth]], light rays can travel large distances, including from beyond the horizon. This was observed and documented in 1596, when a ship in search of the [[Northeast passage]] became stuck in the ice at [[Novaya Zemlya]], above the [[Arctic Circle]]. The Sun appeared to rise two weeks earlier than expected; the real Sun was still visible below the horizon, but its light rays followed the curvature of Earth. This effect is often called a [[Novaya Zemlya effect|Novaya Zemlya mirage]]. For every {{convert|111.12|km}} that light rays travel parallel to 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 that are so far away that they should not be visible above the geometric horizon may appear on or even above the horizon as superior mirages.<ref name="Sample 2021">{{cite news |last1=Sample |first1=Ian |title=Walker 'stunned' to see ship hovering high above sea off Cornwall |url=https://www.theguardian.com/science/2021/mar/05/ship-hovering-above-sea-cornwall-optical-illusion |access-date=6 March 2021 |work=[[The Guardian]] |date=5 March 2021}}</ref> 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.

[[File:Mirages marins.jpg|thumb|Illustration in a 1872 book to describe a mirage]]

If the vertical temperature gradient is +{{convert|12.9|C-change}} per {{convert|100|m|ft|abbr=off|sp=us|disp=x|/}} (where the positive sign means the temperature increases at higher altitudes) then horizontal light rays will just follow the curvature of Earth, and the horizon will appear flat. If the gradient is less (as it almost always is), the rays are not bent enough and get lost in space, which is the normal situation of a spherical, convex "horizon".

In some situations, distant objects can be [[Looming, Towering, Stooping, and Sinking|elevated or lowered, stretched or shortened]] with no mirage involved.

===Fata Morgana===
{{main|Fata Morgana (mirage)}}
A ''Fata Morgana'' (the name comes from the Italian translation of [[Morgan le Fay]], the fairy, shapeshifting half-sister of King Arthur) is a very complex superior mirage. It appears with alternations of compressed and stretched areas, erect images, and inverted images.<ref name="An Introduction to Mirages"/> A Fata Morgana is also a fast-changing mirage.

Fata Morgana mirages are most common in [[polar region]]s, especially over large sheets of ice with a uniform low temperature, but they can be observed almost anywhere. In polar regions, a Fata Morgana may be observed on cold days; in desert areas and over oceans and lakes, a Fata Morgana may be 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 [[Earth]].<ref name="An Introduction to Mirages">{{cite web | last=Young | first=Andrew T. | title=An Introduction to Mirages | website=San Diego State University | url=https://aty.sdsu.edu/mirages/mirintro.html | access-date=2025-11-19 }}</ref>

The rays will bend and form [[Arc (geometry)|arcs]]. An observer needs to be within an [[atmospheric duct]] to be able to see a Fata Morgana.<ref name="Atmospheric Optics Glossary">{{cite web | last=Young | first=Andrew T. | title=Atmospheric Optics Glossary | website=San Diego State University | url=http://mintaka.sdsu.edu/GF/glossary.html#duct | access-date=2025-11-19 }}</ref>
Fata Morgana mirages may be observed from any [[altitude]] within [[Atmosphere of Earth|Earth's atmosphere]], including from mountaintops or airplanes.<ref name=" `SUPERIOR MIRAGE of cumulus cloudtop from airplane">{{cite journal | title=An unusual refractive phenomenon seen from a high-flying aircraft | journal=Meteorological Magazine | volume=85 | last1=Durst | first1=C. S. | last2=Bull | first2=G. A. | year=1956 | pages=237–242 }}</ref><ref name="Annotated bibliography of mirages, green flashes, atmospheric refraction, etc.">{{cite web | title=Annotated bibliography of mirages, green flashes, atmospheric refraction, etc. | last=Young | first=Andrew T. | website=San Diego State University | url=http://mintaka.sdsu.edu/GF/bibliog/bibliog.html | access-date=2025-11-19 }}</ref>

Distortions of image and bending of light can produce spectacular effects. In his book ''Pursuit: The Chase and Sinking of the "Bismarck"'', Ludovic Kennedy describes an incident that allegedly took place below the Denmark Strait during 1941, following the sinking of the ''Hood''. The ''[[German battleship Bismarck|Bismarck]]'', 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 ''Bismarck'' had, in fact, made no change to her course.
{{Multiple image
 | direction     = horizontal
 | total_width   = 500
 | align         = centre
 | caption_align = center
 | image1        = Farallons islands miragep.jpg
 | caption1      = Sequence of a Fata Morgana of the [[Farallon Islands]] as seen from [[San Francisco]]
 | image2        = Farallons islands miragep animated.gif
 | caption2      = The same sequence as an animation
}}

==Night-time mirages==
The conditions for producing a mirage can occur at night as well as during the day.  Under some circumstances mirages of astronomical objects and mirages of lights from moving vehicles, aircraft, ships, buildings, etc. can be observed at night.{{sfn|Lynch|Livingston|Livingston|2001}}

===Mirage of astronomical objects===
{{main|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]].  Mirages can be observed for such astronomical objects as the [[Sun]], the [[Moon]], the [[planet]]s, bright [[star]]s, and very bright [[comet]]s. The most commonly observed are [[sunset]] and [[sunrise]] mirages.

==See also==
* [[Atmospheric refraction]]
* [[Looming and similar refraction phenomena]]

==Notes==
{{reflist|group=lower-alpha}}

==References==
{{reflist}}

==Bibliography==
*{{Cite book|url={{GBurl|id=4Abp5FdhskAC}}|title=Color and Light in Nature|last1=Lynch|first1=David K.|last2=Livingston|first2=William Charles|last3=Livingston|first3=William|date=2001-06-11|publisher=Cambridge University Press|isbn=0-521-77504-3}}

==External links==
{{wikt}}
{{commons category|Mirages}}
{{EB1911 Poster|Mirage}}
* [http://mintaka.sdsu.edu/GF/mirages/mirintro.html All kind of mirages explained]
* [http://www.chinadaily.com.cn/photo/2006-05/08/content_583947.htm China daily, rare mirage in Penglai]
* [https://www.theguardian.com/science/2021/mar/05/ship-hovering-above-sea-cornwall-optical-illusion The superior mirage]
* [http://www.islandnet.com/~see/weather/elements/infmrge.htm The inferior mirage]
* [https://web.archive.org/web/20060218195616/http://www.sas.org/tcs/weeklyIssues/2004-09-10/gallery/index.html The highway mirage]
* [https://bigskywalker.com/2016/12/01/fata-morgana-from-the-cdt/ Fata Morgana Mirage from the Continental Divide Trail]
* {{cite web|last=Bowley|first=Roger|title=Mirages|url=http://www.sixtysymbols.com/videos/mirages.htm|work=Sixty Symbols|publisher=[[Brady Haran]] for the [[University of Nottingham]]|year=2009}}

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[[Category:Atmospheric optical phenomena]]
[[Category:Optical illusions]]


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