What Is a Mirage?
Mirages are optical illusions produced by extraordinary atmospheric conditions. They can be non-existent sheets of water or similar visions, inverted or oversized images of distant objects or various other distortions. All are caused by the refraction, or bending, of light rays as they pass through two layers of air with different densities. The differences in density are usually due to unequal distributions of temperature in the atmosphere.
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. A famous mirage known as the Fata Morgana can be seen in the Strait of Messina, between Italy and Sicily. Here the distorted images of houses on the opposite cliffs are transformed into imaginary castles in sea and sky. The Italians named this mirage after the Fata (or Fairy) Morgana, a legendary enchantress with the magical power of raising phantom castles from the waters.
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.
A common type of mirage is seen in deserts where the heat of the sand raises the temperature of the lower air to make it substantially less dense than that immediately above. This bends the light to such an extent that images of the sky are seen on the ground resembling patches of water. The phenomenon is known as an inferior image. If the layers of hot and cold air are reversed, the result may be a superior mirage, where distorted images of distant objects on the ground are observed in the sky. In this kind of mirage there are sometimes two images, the lower one being upside down.
An inferior mirage is called “inferior” because the mirage is located under 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 in the distance.
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, 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.
Heat haze, also called heat shimmer, refers to the inferior mirage experienced when viewing objects through a layer of heated air; for example, viewing objects across hot asphalt or through the exhaust gases produced by jet engines. When appearing on roads due to the hot asphalt, it is often referred to as a highway 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 creates a gradient in the refractive index of the air. This produces a blurred shimmering effect, which affects the ability to resolve objects, the effect being increased 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 reflected by the road’s surface. The mind interprets this as a pool of water on the road, since water also reflects the sky. The illusion fades as one gets closer.
On tarmac roads it may look as if water, or even oil, has been spilled. These kinds of inferior mirages are often called “desert mirages” or “highway mirages”. 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 create conditions suitable for the formation of the mirage. Heat haze is not related to the atmospheric phenomenon of haze.