How are mirages produced?: Information Roundup
A mirage is an optical effect or atmospheric illusion produced by the irregular refraction and reflection of light rays as they pass through layers of air of unequal density. This phenomenon, observed since ancient times, is especially common in hot desert regions and was first brought to the attention of the scientific world and correctly explained by a French physicist named Gaspard Mange who accompanied Napoleon to Egypt and who described the mirages seen in the desert by the French soldiers. Mirage is derived from Latin mirare, "to look at" or "to wonder at," which is also the ultimate source of miracle, mirror and admiration. All mirages cannot be explained in the same manner and some of them are not entirely understood. Different effects are produced by the atmospheric conditions peculiar to different stretches of land and sea. The simplest and most common mirage is the illusion of a sheet of water where no water exists. Travelers in the desert often see imaginary pools and lakes ahead that disappear or recede as the observer proceeds in their direction. In 1940 it was reported that two duck hunters of Pueblo, Colorado, spent several early morning hours beside a "lake" before they discovered that it was only a mirage. Although the hunters were fooled by the mirage, the ducks were not! Desert sand radiates heat that causes a layer of air close to the ground to expand and become rarer than [page 53] the layers above. The usual condition is thus reversed and the denser stratum lies above the rarer stratum. A distant object viewed across such a blanket of heated air will be seen by means of two different sets of light rays. The more horizontal rays pass close to the ground and assume a path convex in respect to the horizon, while the more oblique rays pass higher up and assume a path concave in respect to the horizon. To an observer above the rarefied layer the object appears double, an inverted image being seen as if mirrored by the sand. Actually it is the layer of heated and rarefied atmosphere that serves as a reflector. Objects are reflected from the sky, being sometimes recognizable and sometimes grotesquely distorted beyond recognition. Mirages of cities and mountains are occasionally seen at great distances from the real objects. Beautiful lakes, surrounded by trees and cliffs, appear in barren deserts. Mountains hundreds of miles away have been seen reflected in the sky at night by lightning. Not infrequently mirages are distinguishable from the actual objects only by their quivering and changing position, and even experienced travelers in the desert are occasionally misled by these optical illusions. It is possible but difficult to photograph a clearly visible mirage. A camera is essentially like the eye and it sees what the eye sees, although it is not so sensitive to color and does not register all that the eye does. Miniature mirages occur all around us. They can be seen over hot stoves and the tops Of sun-heated cliffs. Motorists driving over paved roads on hot days appear to see pools or sheets of water, with other objects reflected in them, in the road ahead, but the pools are only imaginary and vanish when the car approaches the places where they seem to be. The pavement, like the desert sand, heats the layer of air next to it and converts it into a mirror. It is an interesting fact that mirages, once common on the prairies and plains west of the Mississippi, are now seldom seen except in certain sections, cultivation of the soil having produced conditions unfavorable to these optical illusions. At sea the denser and rarefied layers of atmosphere are often reversed and a different type of mirage results. The denser layer is near the water and the reflection occurs in the rarer atmosphere above the observer. Images of ships and icebergs actually below the horizon sometimes appear inverted and suspended in the air, frequently being multiplied and magnified. When bodies of air of different densities are in close proximity weird effect may be produced. A single vessel below the horizon may appear as a long row of exceedingly tall vessels. Again, a ship viewed over a misty or foggy body of water may appear to be elevated and elongated, a phenomenon known as looming or towering. Mirages [page 54] not only produce grotesque, weird and fantastic effects but play queer pranks. During the First World War these optical illusions interfered with the fighting between the British and Turks in Mesopotamia Targets were distorted and armies were concealed, magnified or placed in imaginary situations by mirages. In 1906, Admiral Robert E. Peary, standing on the northernmost point of Axel Heiberg, thought he saw a vast land in the Arctic sea that was later indicated on maps Crockerland. The MacMillan expedition eight years later sought in vain for this territory. It was in fact an Arctic mirage and no such land ever existed. Occasionally the successive layers of atmosphere bear such a relation to one another that the whole acts as a gigantic lens and brings distant objects in large proportions to the observer. A classic case of this type of mirage is the Fata Morgana. In the latter part of the eighteenth century a Dominican friar named Antonio Minasi wrote a description of a fantastic mirage that he saw across the Strait of Messina in southern Italy. The friar saw a land of enchantment pass rapidly before him, a magnificent city with castles, palaces, spired cathedrals, towers with battlements, mighty obelisks, spacious gardens and expansive parks with perfectly spaced trees. He called it Fata Morgana, Italian for "Morgan the Fay" or "Fairy Morgan," the name of King Arthur's sister in the Arthurian legends. Since the Normans of England believed that Morgan the Fay, who was a necromancer, lived in Calabria near the Strait of Messina, Friar Antonio named after her the magic city that he had seen.