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A mirage is a naturally-occurring
optical phenomenon Optical phenomena are any observable events that result from the interaction of light and matter. All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the Sun or Moon with ...
in which light rays bend via
refraction In physics, refraction is the redirection of a wave as it passes from one transmission medium, medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commo ...
to produce a displaced image of distant objects or the sky. The word comes to English via the French ''(se) mirer'', from the
Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
''mirari'', meaning "to look at, to wonder at". Mirages can be categorized as "inferior" (meaning lower), "superior" (meaning higher) and " 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 hallucination is a perception in the absence of an external stimulus that has the compelling sense of reality. They are distinguishable from several related phenomena, such as dreaming ( REM sleep), which does not involve wakefulness; pse ...
, 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

In an inferior mirage, the mirage image appears below the real object. The real object in an inferior mirage is the (blue)
sky The sky is an unobstructed view upward from the planetary surface, surface of the Earth. It includes the atmosphere of Earth, atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from ...
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. Light rays coming from a particular distant object all travel through nearly the same layers of air, and all are
refracted In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...
at about the same
angle In Euclidean geometry, an angle can refer to a number of concepts relating to the intersection of two straight Line (geometry), lines at a Point (geometry), point. Formally, an angle is a figure lying in a Euclidean plane, plane formed by two R ...
. 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 An illusion is a distortion of the senses, which can reveal how the mind normally organizes and interprets sensory stimulation. Although illusions distort the human perception of reality, they are generally shared by most people. Illusions may ...
that the sky image seen in the distance is a
specular reflection Specular reflection, or regular reflection, is the mirror-like reflection (physics), reflection of waves, such as light, from a surface. The law of reflection states that a reflected ray (optics), ray of light emerges from the reflecting surf ...
on a puddle of water or oil acting as a
mirror A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...
. 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 denser) sinks, the layers will mix, causing
turbulence In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disruption between ...
. The image will be distorted accordingly; it may vibrate or be stretched vertically ( towering) or compressed vertically ( stooping). A combination of vibration and extension are also possible. If several 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 The angular diameter, angular size, apparent diameter, or apparent size is an angular separation (in units of angle) describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the ''visual an ...
of the Sun and Moon) and are from objects between dozens of meters and a few kilometers away. File:Desertmirage.jpg, An inferior mirage seen in the
Mojave Desert The Mojave Desert (; ; ) is a desert in the rain shadow of the southern Sierra Nevada mountains and Transverse Ranges in the Southwestern United States. Named for the Indigenous peoples of the Americas, indigenous Mohave people, it is located pr ...
in a Nevada spring File:Inferior Mirage Grand Traverse Bay.jpg, An inferior mirage over
Grand Traverse Bay Grand Traverse Bay ( ) is an arm of Lake Michigan, located along the west coast of the Lower Peninsula of Michigan. The bay is separated from the rest of Lake Michigan by the Leelanau Peninsula. The bay is some long, ranges from wide, and up ...
in
Michigan Michigan ( ) is a peninsular U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest, Upper Midwestern United States. It shares water and land boundaries with Minnesota to the northwest, Wisconsin to the west, ...


Heat haze

''Heat haze'', also called ''heat shimmer'', refers to the inferior mirage observed when viewing objects through a
mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...
of heated air. Common instances when heat haze occurs include images of objects viewed across
asphalt concrete Asphalt concrete (commonly called asphalt, blacktop, or pavement in North America, and Tarmacadam, tarmac or bitumen macadam in the United Kingdom and the Republic of Ireland) is a composite material commonly used to surface road surface, roads ...
(also known as tarmac), roads and over masonry rooftops on hot days, above and behind fire (as in burning
candle A candle is an ignitable candle wick, wick embedded in wax, or another flammable solid substance such as tallow, that provides light, and in some cases, a Aroma compound, fragrance. A candle can also provide heat or a method of keeping time. ...
s,
patio heater A patio heater, also called a mushroom heater or umbrella heater, is a radiant heating appliance for generating thermal radiation for outdoor use. Types of heater A burner on top of a pole, it burns natural gas, liquefied petroleum gas (LPG), pro ...
s, and
campfire A campfire is a fire at a campsite that provides light and warmth, and heat for cooking. It can also serve as a beacon, and an insect and predator deterrent. Established campgrounds often provide a stone or steel fire ring for safety. Campfires ...
s), and through
exhaust gas Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through ...
es from
jet engine A jet engine is a type of reaction engine, discharging a fast-moving jet (fluid), jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition may include Rocket engine, rocket, Pump-jet, water jet, and ...
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 higher than the air above, enough to make conditions suitable to cause the mirage.
Convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...
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 In vector calculus, the gradient of a scalar-valued differentiable function f of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p gives the direction and the rate of fastest increase. The g ...
in the
refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
of the air. This produces a blurred shimmering effect, which hinders the ability to resolve the image and increases when the image is magnified through a
telescope A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
or
telephoto lens A telephoto lens, also known as telelens, is a specific type of a long-focus lens used in photography and cinematography, in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens ...
. Light from the sky at a shallow angle to the road is
refracted In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...
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 Haze is traditionally an atmospheric phenomenon in which dust, smoke, and other dry particulates suspended in air obscure visibility and the clarity of the sky. The World Meteorological Organization manual of codes includes a classificati ...
.


Superior mirage

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 In meteorology, an inversion (or temperature inversion) is a phenomenon in which a layer of warmer air overlies cooler air. Normally, air temperature gradually decreases as altitude increases, but this relationship is reversed in an inver ...
. 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''. Superior mirages are quite common in
polar region The polar regions, also called the frigid zones or polar zones, of Earth are Earth's polar ice caps, the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitu ...
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, or ''hafgerðingar'' in the
Icelandic language Icelandic ( ; , ) is a North Germanic languages, North Germanic language from the Indo-European languages, Indo-European language family spoken by about 314,000 people, the vast majority of whom live in Iceland, where it is the national languag ...
. 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 of Earth Spherical Earth or Earth's curvature refers to the approximation of the figure of the Earth as a sphere. The earliest documented mention of the concept dates from around the 5th century BC, when it appears in the writings of Greek philosophers. ...
, 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 The Northeast Passage (abbreviated as NEP; , ) is the Arctic shipping routes, shipping route between the Atlantic Ocean, Atlantic and Pacific Ocean, Pacific Oceans, along the Arctic coasts of Norway and Russia. The western route through the islan ...
became stuck in the ice at
Novaya Zemlya Novaya Zemlya (, also , ; , ; ), also spelled , is an archipelago in northern Russia. It is situated in the Arctic Ocean, in the extreme northeast of Europe, with Cape Flissingsky, on the northern island, considered the extreme points of Europe ...
, above the
Arctic Circle The Arctic Circle is one of the two polar circles, and the northernmost of the five major circle of latitude, circles of latitude as shown on maps of Earth at about 66° 34' N. Its southern counterpart is the Antarctic Circle. The Arctic Circl ...
. 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 mirage. For every 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. 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 + per (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 elevated or lowered, stretched or shortened with no mirage involved.


Fata Morgana

A ''Fata Morgana'' (the name comes from the Italian translation of
Morgan le Fay Morgan le Fay (; Welsh language, Welsh and Cornish language, Cornish: Morgen; with ''le Fay'' being garbled French language, French ''la Fée'', thus meaning 'Morgan the Fairy'), alternatively known as Morgan , Morgain /e Morgant Mor ...
, 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. A Fata Morgana is also a fast-changing mirage. Fata Morgana mirages are most common in
polar region The polar regions, also called the frigid zones or polar zones, of Earth are Earth's polar ice caps, the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitu ...
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 In meteorology, an inversion (or temperature inversion) is a phenomenon in which a layer of warmer air overlies cooler air. Normally, air temperature gradually decreases as altitude increases, but this relationship is reversed in an inver ...
has to be strong enough that light rays' curvatures within the inversion are stronger than the curvature of
Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...
.Young, Andy
An Introduction to Mirages
The rays will bend and form arcs. An observer needs to be within an
atmospheric duct In telecommunications, an atmospheric duct is a horizontal layer in the lower atmosphere in which the vertical refractive index gradients are such that radio signals (and light rays) are guided or ducted, tend to follow the curvature of the Ear ...
to be able to see a Fata Morgana.Young, Andy
SDSU.edu
"Atmospheric Optics Glossary".
Fata Morgana mirages may be observed from any
altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...
within
Earth's atmosphere The atmosphere of Earth is composed of a layer of gas mixture that surrounds the Earth's planetary surface (both lands and oceans), known collectively as air, with variable quantities of suspended aerosols and particulates (which create weathe ...
, including from mountaintops or airplanes. 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 '' 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.


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.


Mirage of astronomical objects

A mirage of an astronomical object is a naturally occurring
optical phenomenon Optical phenomena are any observable events that result from the interaction of light and matter. All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the Sun or Moon with ...
in which light rays are bent to produce distorted or multiple images of an
astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists within the observable universe. In astronomy, the terms ''object'' and ''body'' are of ...
. Mirages can be observed for such astronomical objects as the
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
, the
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
, the
planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...
s, bright
star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...
s, and very bright
comet A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...
s. The most commonly observed are
sunset Sunset (or sundown) is the disappearance of the Sun at the end of the Sun path, below the horizon of the Earth (or any other astronomical object in the Solar System) due to its Earth's rotation, rotation. As viewed from everywhere on Earth, it ...
and
sunrise Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning, at the start of the Sun path. The term can also refer to the entire process of the solar disk crossing the horizon. Terminology Although the S ...
mirages.


See also

*
Atmospheric refraction Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of height. This refraction is due to the velocity of light ...
*
Looming and similar refraction phenomena While mirages are the best known atmospheric refraction phenomena, looming and similar refraction phenomena do not produce mirages. Mirages show an extra image or images of the miraged object, while looming, towering, stooping, and sinking do not. ...


Notes


References


Bibliography

*


External links


All kind of mirages explained



The superior mirage





Fata Morgana Mirage from the Continental Divide Trail
* {{Authority control Atmospheric optical phenomena Optical illusions de:Luftspiegelung