Correct Option: D

Mirage is caused by total internal reflection in deserts where due to heating of the earth, refractive index of air near the surface of earth becomes lesser than above it. A ray of light coming from a distant object gets refracted from a denser to a rarer medium. Consequently the refracted ray bends away from the normal until at a particular layer, the light is incident at an angle greater than the critical angle. At this stage the incident ray suffers total internal reflection and is reflected upwards. When this reflected beam of light enters the eyes of the observer, it creates an illusion of the water near the object.