Mirage is an optical illusion due to total internal reflection. It is generally observed in deserts or on road during sunny days. It is caused by refraction of light due to different temperatures of the layers of the air which results in the illusion of water on the ground.

Mirage is a phenomenon that occurs when light rays undergo total internal reflection. It is a fascinating optical illusion that is often observed in deserts or on hot roads, where the appearance of water or a pool seems to be present, even though there is no actual water.

Total Internal Reflection:
- Total internal reflection occurs when light travels from a denser medium to a less dense medium at an angle of incidence greater than the critical angle.
- In the case of a mirage, the denser medium is the hot air closer to the ground, and the less dense medium is the cooler air above it.
- When light passes from the hot air near the ground to the cooler air above it, it bends away from the normal due to the change in refractive index.
- As the angle of incidence increases, at a particular angle, known as the critical angle, the refracted ray bends to an angle of 90 degrees, parallel to the interface between the two mediums.
- Beyond the critical angle, light undergoes total internal reflection, bouncing back into the denser medium instead of refracting out into the less dense medium.

Formation of Mirage:
- In a desert or on a hot road, the air near the ground gets heated up, creating a temperature gradient.
- This temperature gradient causes the refractive index of air to decrease with increasing height above the ground.
- When light from the sky or distant objects passes through this temperature gradient, it undergoes refraction due to the varying refractive index.
- The light rays coming from the sky or distant objects are bent away from the normal, creating an inverted image of the sky or objects.
- This inverted image appears as a pool of water, giving the illusion of a mirage.

Explanation of Options:
a) Reflection of light: Reflection of light does occur in a mirage, but it is not the primary phenomenon responsible for its formation.
b) Refraction of light: Refraction of light is involved in the formation of a mirage, but it is not the sole reason.
c) Total internal reflection of light: Total internal reflection is the main phenomenon responsible for the formation of a mirage, as explained above.
d) Diffraction of light: Diffraction of light does not play a significant role in the formation of a mirage.

In conclusion, a mirage is caused by the total internal reflection of light due to a temperature gradient in the air near the ground. This optical illusion creates the appearance of water or a pool, even though there is no actual water present.