Explain the formation of a mirage.

Mirage : It is an optical illusion observed in deserts or hot summer days due to which a person sees a pond of water some distance ahead of him in which the surrounding objects appear inverted. It is caused due to total internal reflection.

i) The temperature of air layers goes on decreasing as you go up. Therefore, the density and hence the refractive index of air layers increase as one goes up.
ii) Thus a ray of light from the top O of a tree passes through air of gradually decreasing refractive index.
iii) As a result, the ray of light gets refracted more and more away from the normal. At a particular layer when the angle of incidence becomes greater than the critical angle, total internal reflection takes place and the ray of light is returned into the same medium i.e. it is reflected upward.
iv) When the reflected ray reaches the eye of the observer, it appears to be coming from I.
v) Hence, the inverted image of the tree creates an impression of reflection from a pond of water.