What is a mirage? Give a reason for its formation.

Sometimes, in deserts, an inverted image of a tree is seen which gives a false impression of water under the tree. This is called a mirage.

Mirage is formed due to the refraction of light. In a desert, during the day-time, the hot sand rapidly heats the layers of air in contact with it. As a result, the layers of air near the ground are rarer and the successive upper layers are denser than those below them.

The light rays from sun after reflection from the top of a tree travel from denser (cold upper layers of air) to rarer medium (warmer lower layer of air) and undergo refraction bending away from the normal. On successive refraction, the angle of refraction increases and the angle of incidence of ray going from denser to rarer medium also increases till a stage is reached when the angle of refraction becomes 90°. On further increase in angle of incidence the light ray is not refracted but is reflected. This reflected ray now travels from rarer to denser layer and bends towards the normal at each refraction. On reaching the eye of the observer, an inverted image of the tree is seen. Thus, it gives a false impression of a pool of water in front of the tree.