Explain total internal reflection. What are its conditions?

Explanation

Formation of mirage: Mirage is an optical illusion observed in deserts on hot sumer days. An observer sees the inverted image (of a distant object such as tree) asif the reflection is taking place from water. This creates the impression of water or pond of water there. But when observer reaches there no water is found actually. THis optical illusion is called mirage.

On still summer days, the air near the ground may become hotter due to rise in temperature than air farther up. Thus the layers of air near the sand become lighter and behaves like rarer medium. The refractive index goes on increasing with increase in height up to a few meters. A ray of light from the top of the tree passes through a medium whose refractive index decreases towards ground and reaches the surface of the earth with angle of incidence greater than critical angle and total internal reflection takes plce. The rays of light, after total internal reflection, enter the observer.s eye travelling along AE. When extended backwards, the ray appears to becoming from I, which behaves like a mirror image of O. This inverted image creates the illusion of water.