Name: Snell's Law
Uploaded: 2019-07-19
Duration: 6 min 19 s
Description: Snell's law relates the angle of incidence and angle of refraction for a pair of media with their refractive indices. Let's understand Snell's law through an...

Question

What is mirage? Give a reason for its formation?

Accepted Answer

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. The cause of the mirage is the refraction of light. In a desert, the sand becomes very hot during the day-time, and it rapidly heats the layers of air in contact with it. Therefore, the layers of air near the ground are warmer (and hence rarer) than the upper layers. In other words, the successive upper layers are denser than those below them.

When a ray of light from the sun after reflection from the top of a tree travels from a denser to a rarer layer, it bends away from the normal. As a result, in refraction at the surface of separation of successive layers, each time the angle of refraction increases and the angle of incidence of ray going from denser medium to rarer medium also increases, till a stage is reached when the angle of refraction becomes $90 °$ . On further increase in the angle of incidence, the ray of light travelling from a denser to a rarer layer is not refracted, but it suffers reflection. This reflected ray now travels from the rarer to the denser layer, so it 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.

What is mirage? Give a reason for its formation?

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