Greenler, Robert Department of Physics, University of Wisconsin, Madison, Wisconsin.
Last reviewed:January 2020
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A distorted scene of distant objects resulting from the passage of light through a nonuniform medium. In a uniform atmosphere light would travel in straight lines and distant scenes would appear without distortion. Since the density of the air changes with height and with temperature, the atmosphere is not uniform and light rays will follow curved paths. The direction of the curvature is to deflect a ray away from a warmer (less dense) layer of air. Figure 1 shows some of the features of the most familiar desert or hot-road mirage. The ground is heated by the sun and in turn heats (and rarifies) the layer of air just above the ground. Then rays passing near the ground are bent (refracted) upward. The observer can see the top of the tree in the figure, for example, by looking in two directions. The strongly refracted rays produce an inverted view of the tree. The only other place in nature where such an effect can be seen is in reflection from a water surface, and it is natural to interpret the mirage as evidence of a body of water.
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