An optical illusion which occurs mainly in deserts during hot summer is based on the principle of

  1. Reflection
  2. Interference
  3. Dispersion
  4. Total internal reflection


The phenomenon is known as Mirage. It is due to Total Internal Reflection. On a very hot day in desert, the air just in contact with sand is hotter than the air above it due to the high temperature of the sand. This causes the refractive index of the air in contact with the sand to be lower than that of the air above it. Therefore, sunlight passes through a medium of higher refractive index to that of a lower refractive index.

At a particular angle, the angle of incidence becomes greater than the critical angle. This causes the sunlight to reflect from the interface of both the air columns itself without touching the sand. Hence, it appears as a watery surface from far off distances. While going nearer to the apparent water, the angle of incidence of sunlight decreases and becomes less than the critical angle. So, total internal reflection does not occur and the mirage disappears.

The correct option is D.

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