Mirage

format_list_bulleted Contenido keyboard_arrow_down
ImprimirCitar
Lower mirage on a road. The distant objects are reflected on a smooth surface as if a liquid surface is being contemplated that does not actually exist.

A mirage is a naturally occurring optical illusion in which light rays are bent through refraction to produce a displaced image of distant objects or the sky. Mirages can be classified as "lower", "superior" and "Fata Morgana", a type of superior mirage consisting of a series of unusually elaborate vertically stacked images, forming a rapidly changing mirage.

Unlike a hallucination, a mirage is an actual optical phenomenon that can be captured with a camera as light rays are refracted to form the false image at the observer's location. However, what the image appears to represent is determined by the interpretative faculties of the human mind. For example, lower mirages are very easily mistaken for reflections from a small body of water.

History

Illustration of a book of the centuryXIX, which shows expanded higher mirages; mirages can never be so above the horizon, and superior mirage can never increase the length of an object as shown on the right.

The first observations of mirages appear to date back to 350 BC. C., when Aristotle mentions in the Meteorology that it happens that the promontories appear abnormally large in certain meteorological circumstances and that the southeast wind makes the stars increase in size when they set or rise4.

"This is also what makes the peaks of the promontories look higher in the sea, and that the dimensions of all objects increase when the southeast wind blows. This is also what happens with the objects that appear through the fog; for example, the sun and stars, when they come out or get laid, look bigger than when they are in the center of the sky.- Aristotle, Meteorological translated by J. Barthélemy Saint-Hilaire, A. Durand Libraire éditeur, 1863

This passage is part of a chapter dealing with the refraction of light and the explanation of the rainbow phenomenon. It is very likely that the deformation suffered by the stars and the enlargement of the "promontories" are due to the refraction of light by the hot air transported by the sirocco, a southeasterly wind that is mentioned.

There is also mention of people disappearing from sight because of the air "densified by heat " or the sun, distorted and surrounded by "flames" 6,7, or indistinct forms, motionless or moving, compared to strange creatures (Diodorus of Sicily in Bibliothèque historique Tome 2).

In the Natural History of Pliny the Elder, a multitude of physical and astronomical phenomena are listed in Book II, among which the existence of multiple Suns visible in the morning and in the afternoon (chapter XXXII), and later8:

"During the third consulate of Mario, the inhabitants of Ameria and Tudertum saw how the heavenly armies came to crash from the east and west, and those on the west side were defeated. Several times the sky itself was seen on fire; this is not surprising: it is the clouds that light up in a large area." - Plinio el Viejo, Histoire naturelle translated by Émile Littré, Firmin Didot et Cie, 1877

This type of armies appearing in the air, clashing and then retreating, is in fact a recurring event in ancient accounts. Flavio Josephus mentions armies that appear in the air in his War of the Jews, the same is described in book II, chapter VII of the Maccabees, appearances of spectral armies summoned by the Huns during the reign of Cariberto I, etc. These violently clashing armies could very well be fey morgana, shimmering, shimmering mirages appearing on the horizon.

Throughout history mirages are mentioned sporadically. In 1799, Gaspard Monge devoted part of his account of the Egyptian campaign to talking about the mirages that he had observed in the desert. He describes the phenomenon of mirage in a qualitative and quite precise way, that is, no longer like Aristotle -refraction in air densified by water vapor- but as a phenomenon of refraction in air densified by temperature, with a total reflection that makes the rays follow a curve.

Mechanism

Desert mirage1874.

In tropical deserts, the air in contact with the torrid ground heats up and its density varies in such a way that, contrary to usual, the colder air stays above the hotter one, which is heated by radiation reflected by the ground. This creates an uneven density in the air which gives it various refractive indices. Therefore, a ray of light reflected by a distant object that goes downwards, and in the direction of the observer, undergoes successive refractions when going through the different layers of air; its inclination towards the ground is less and less and, after reaching the horizontal, the ray undergoes new refractions, although this time upwards. This is how, after having described a curved trajectory of convexity directed downwards, it reaches the eye of the observer, who sees on the ground (inferior mirage) an unclear image of the object. However, as other rays from a real source also reach the observer's eye directly, he has the impression of seeing the object at the same time (for example, a palm tree in a desert) and, at the bottom of it, a second inverted image, as if this palm tree were reflected in a non-existent liquid surface. Therefore, in the hottest hours of summer, the image of the sky appears to come from the asphalt of the hot road, at the same time that it appears wet or puddled to the observer.

It is exceptional that the trajectory of light rays is convex upwards (superior mirage); if it occurs, a ship, a mountain, etc., appear to float in the atmosphere.

Lesser Mirage

An inferior mirage seen in the Mojave Desert in spring in Nevada
Mechanism of a lower mirage. The uneven density of the air, due to certain temperatures, produces a refraction of the light of the sky. This in turn causes the observer to interpret his origin from the ground. This results in an illusion in which a pool of water appears that reflects the light of the sky.

In an inferior mirage, the image of the mirage appears below the actual object. The actual object in an inferior mirage is the (blue) sky or any distant (thus bluish) object in that same direction. The mirage causes the observer to see a bright bluish patch on the ground.

Light rays coming from a particular distant object travel through almost the same layers of air and are all refracted at roughly the same angle. Therefore, the rays that come from the top of the object will arrive lower than those from the bottom. The image is often upside down, increasing the illusion that the image of the sky seen in the distance is a specular reflection in a pool of water or oil that acts as a mirror.

The images below are not stable. As warmer air rises while cooler air (which is more dense) sinks, the layers will mix and cause turbulence. The image will be distorted accordingly; it can vibrate or extend vertically (soaring) or horizontally (curved). A combination of vibration and extension is also possible. If multiple temperature layers are present, multiple mirages may mix together, perhaps causing double images. In any case, mirages are usually no more than half a degree tall (roughly the angular diameter of the Sun and Moon) and come from objects tens of meters to a few kilometers away.

Hot air

A hot air mirage, in which "false water" appears on the road, is the most commonly observed example of a lower mirage.
Hot air mirage seen through the exhaust gases of a reaction motor
Effects of a lower mirage on the sunset. The Sun appears invested near the horizon.

Hot air mirage, also called heat glow, refers to the inferior mirage observed when viewing objects through a mass of hot air. Common instances where this type of mirage occurs include images of objects seen through asphalt roads and over masonry roofs on hot days, above and behind fires (such as lighted candles, patio heaters, and campfires), and through of exhaust gases from jet engines. When it appears on roads due to hot asphalt, it is often referred to as a 'highway mirage'. It also occurs in deserts; in that case, it is known as a "desert mirage." Both asphalt and sand can get very hot when exposed to the sun, easily reaching more than 10°C above air temperature one meter above, enough for conditions to be right for the mirage.

Convection causes the air temperature to vary, and the variation between the warm air at the road surface and the denser cold air above causes a gradient in the refractive index of the air. This produces a blurry glow effect, which hinders the ability to resolve the image and increases when the image is magnified through a telescope or telephoto lens.

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 reflecting off the road surface. The mind interprets this as a pool of liquid (usually water, but possibly others, such as oil) on the road, since some types of liquid also reflect the sky. The illusion vanishes when the observer approaches the mirrored object.

Superior Mirage

Mechanism of superior mirage (Fata Morgana). The object — in this case a sailboat — seems to float in the air.
Upstairs: a superior mirage of an ice plane, McMurdo station
Down: an artificial mirage, which uses sugar solutions to simulate the investment layers.

A superior mirage is one in which the image of the mirage appears to be located above the actual object. A superior mirage occurs when the air below the line of sight is cooler than the air above it. This unusual arrangement is called a temperature inversion, since warm air above cold air is the opposite of the normal temperature gradient of the atmosphere during the day. Going through the temperature reversal, the light rays are tilted downward, so the image appears above the true object, hence the name superior. Superior mirages tend to be more stable than inferior mirages, since they cold air has no tendency to rise and warm air has no tendency to fall.

Upper mirages are quite common in the polar regions, especially over large ice sheets that have a uniformly low temperature. Upper mirages also occur at more moderate latitudes, although in those cases they are weaker and tend to be less smooth and stable. For example, a distant shoreline may look like a tower and appear taller (and therefore perhaps closer) than it really is. Due to the turbulence, there appear to be dancing peaks and towers. This type of mirage is also called Fata Morgana or hafgerðingar in the Icelandic language.

A superior mirage can be right side up or right side up, depending on the distance from the true object and the temperature gradient. Often the image appears as a distorted mix of up and down parts.

Effects of superior mirage on the sunset.

Since the Earth is round, if the downward curvature of light rays is approximately the same as the curvature of the Earth, light rays can travel great distances, even from beyond the horizon. This was observed and documented in 1596, when a ship seeking the northeast passage got stuck in the ice at Novaya Zemlya, above the Arctic Circle. The Sun seemed to rise two weeks earlier than expected; the real Sun was still below the horizon, but its light rays followed the curvature of the Earth. This effect is often called a Novaya Zemlya mirage. For every 111.12 kilometers that light rays travel parallel to the Earth's surface, the Sun will appear 1° higher on the horizon. The inversion layer must have the proper temperature gradient over the entire distance for this to be possible.

In the same way, ships that are so far away that they should not be visible above the geometric horizon can appear on or even above the horizon as superior mirages. This may explain some stories about flying ships or cities coastlines 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 +12.9 °C per 100 meters (where the positive sign means that the temperature increases at higher altitudes), the horizontal light rays will follow the curvature of the Earth and the horizon it will still look flat. If the gradient is smaller (as it almost always is) the rays don't bend enough and get lost in space, which is the normal situation for a "horizon" spherical and convex.

In some situations, distant objects can be raised or lowered, stretched or shortened without any mirage.

Fate Morgana

A Fata Morgana (the name comes from the Italian translation of Morgan le Fay, the fairy, shapeshifting half-sister of King Arthur) is a highly complex Higher Mirage. It appears with alternating compressed and stretched areas, erect images, and inverted images. [6] A Fata Morgana is also a rapidly changing mirage.

Fata Morgana mirages are most common in the polar regions especially over large ice sheets with a uniformly low temperature, but can be observed almost anywhere. In the polar regions, a Fata Morgana can be observed on cold days; in desert areas and over oceans and lakes, a Fata Morgana can be observed on hot days. For Fata Morgana, the temperature inversion must be strong enough so that the bends of the light rays within the inversion are stronger than the curvature of the Earth. [6] [verification required]

The rays will bend and form arcs. An observer must be inside an atmospheric duct to be able to see a Fata Morgana. [7] The Fata Morgana mirages can be observed from any altitude within the Earth's atmosphere including from mountaintops or from aircraft.

Image distortions and light bending can produce dramatic effects. In his book Pursuit: The Chase and Sinking of the & # 34;Bismarck & # 34; Ludovic Kennedy describes an incident that supposedly took place below the Denmark Strait during 1941, after the Hood sank. The Bismarck, pursued by the British cruisers Norfolk and Suffolk, disappeared from sight in a sea fog. Within seconds the ship reappeared sailing towards the British ships at high speed. Alarmed, the cruisers parted, anticipating an imminent attack, and observers on both ships watched in amazement as the German battleship fluttered, faded, and vanished. Radar surveillance during these events indicated that the Bismarck had not, in fact, made any course changes.

Fata Morgana Sequence on the Farallón Islands views from San Francisco
Same animated sequence
Más resultados...
Tamaño del texto:
undoredo
format_boldformat_italicformat_underlinedstrikethrough_ssuperscriptsubscriptlink
save