What is a mirage? A mirage is a misleading appearance. Most mirages occur on the seas or in the deserts. What will cause a mirage? A reflection. What causes reflection? Light. We seldom consider light as anything magical or wonderful, but light allows us the ability to see many good things and, often, many bad things.
Mirages, also called illusions, are caused by a reflection of some distance object which allows you to think that it is close by. In physics, it is known as an optical illusion. The more common type of mirage is called inferior mirage. It happens when a refraction of light passes through the atmosphere layers with varying qualities. Distance objects may seem to be raised above or below their normal locality. These objects may be seen as irregular and fantastic shapes.
In warmer climates, such as deserts and sandy plains, mirages frequently occur. It normally comes in the appearance of the desert resembling a sheet of water, especially if you are somewhat higher than the mirage you experience. If you've been driving down the interstate in the heat of summer, you probably have noticed this same effect. With this case, the image is really of the sky. How does it occur? If you are below eye level of this surface, all objects will appear inverted, or upside-down. Over a hot surface, air will stand in layers that are of a different element or part. The layers below or near the ground are the hottest. These air layers cause a distortion of wave fronts, due to the speed of light varying as the element or parts change.
Superior mirages are spectacular events, but much less common than the inferior mirage. These occur mainly over the horizon of the sea when distant objects are sketched, or drawn, upside down n the sky. Sometimes there is an erect image of the same object which will be above the upside-down image. This is characteristic of cold areas and conditions with a strong change of temperature where the warmer layers of air rise above the cooler layers. This involves a complicated action of wave fronts of light as the pass through the layers. Within the polar regions, these shapes may take the form of horrible and unusual images. If you see the mirage on land, the trees and other landscape objects will be turned upside down and these images are always clearly defined.
A lateral mirage can be seen when 2 layers of air are separated by a vertical (straight up and down) plane. This type of mirage takes place at a south facing wall, within the Northern hemisphere, which has absorbed considerable heat.
Some mirages have specific names:
1. Looming - appearance of objects usually hidden below the horizon. Normally occur over water surfaces when normal rate of air thickness decreases and altitude is heightened.
2. Sinking - reverse effect of the above phenomenon. Occurs when the opposite conditions at sea take place. In sinking, the vessels, boats and shorelines which are seen on the horizon, seem to sink below and become invisible.
3. Towering - occurs due to irregular refraction. Light rays curve downward, with the top of the object curving more than the lower ones. The observer will see objects which seem to be lifted up more then they need to be and will be enlarged in the vertical direction.
4. Stooping - when the light rays of the distant object curve downward less than the rays at the bottom. This vertical contraction gives it this name. It results in objects on the horizon being observed with the rising or setting of the sun and the moon. One may often see a distortion caused by irregular layer effects of the lower atmosphere strata. One of the most famous of these occurs between Calabria and Sicily and is known as fata morgana.
Light has many wonderful and surprising qualities. It is easy to become so familiar with everyday things that we forget we have much to learn and discover. One of Aesop's fables says, "Familiarity breeds contempt." Sometimes when we become so used to things in our lives, we take them for granted and, usually try to ignore them. Why not take a new look at the moonlight or the morning sunrise. You may discover the next frontier.
For more information and demonstrations on mirages, visit SandlotScience. They have over 100 illusions to view, plus e-cards to send to your friends.
1. Kennon, William L. Astronomy: A Textbook for Colleges. Ginn and Company: Boston. 1968.
2. Editors. The World Book Encyclopedia. World Book-Childcraft International, Inc: Chicago. 1990