What is a mirage?

Refraction Read more from
Chapter Light

The portion of a person’s body that is above water does not appear out of proportion because the light entering your eye is not going through a different medium and refracting. The part of the body that is underwater, the person’s legs, appears to be short because the light reflecting off their legs is traveling through water and then into the air. Due to this change in medium, refraction occurs. Since the index of refraction for water is larger than the index of refraction for air, the legs of the person appear compressed and stocky.

Mirages typically occur on hot summer days when surfaces such as sand, concrete, or asphalt are warm. Mirages look like pools of water on the ground, along with an upside-down image of a building, vehicle, or tree in the distance. As one approaches a mirage, the puddle of water and the reflection seem to disappear.

A mirage occurs because of a temperature difference between the air directly above the surface, which is hot and thus less dense, and the cooler, denser air a few meters above the surface. The denser air has a higher refractive index and that causes the light from an object to bend up toward the observer. As a result, the object is right side up while the refracted image is inverted and underneath the original object. The illusion of water is also a refracted image, the image of the sky. Mirages can only occur on hot surfaces and objects that are at relatively small angles in relation to the observer. Therefore, a person cannot see a mirage from an object that is just a few meters away. A mirage is not a hallucination, but instead a true and well-documented optical phenomenon.


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