Question

Explain the formation of a mirage? ?

Answer 1

A mirage is an optical phenomenon that creates the illusion of water and results from the refraction of light through a non-uniform medium. Mirages are most commonly observed on sunny days when driving down a roadway. As you drive down the roadway, there appears to be a puddle of water on the road several yards (maybe one-hundred yards) in front of the car.
The role of the sun is to heat the roadway to high temperatures. This heated roadway in turn heats the surrounding air, keeping the air just above the roadway at higher temperatures than that day's average air temperature. Hot air tends to be less optically dense than cooler air. As such, a non-uniform medium has been created by the heating of the roadway and the air just above it. While light will travel in a straight line through a uniform medium, it will refract when traveling through a non-uniform medium. If a driver looks down at the roadway at a very low angle (that is, at a position nearly one hundred yards away), light from objects above the roadway will follow a curved path to the driver's eye. Light that is traveling downward into this less optically dense air begins to speed up. Though there isn't a distinct boundary between two media, there is a change in speed of a light wave. As expected, a change in speed is accompanied by a change in direction. If there were a distinct boundary between two media, then there would be a bending of this light ray away from the normal. For this light ray to bend away from the normal (towards the boundary), the ray would begin to bend more parallel to the roadway and then bend upwards towards the cooler air. As such, a person in a car sighting downward at the roadway will see an object located above the roadway.

Answer 2

Formation of mirage is the best example to show where refractive index of a medium changes through out the medium
EXPLANATION:
When air at the top has greater refractive index than hotter air just above the road.When the light from a tall object such as tree or from the sky passes through a medium just above the road whose refractive index decreases towards ground it suffers refraction. This refracted light reaches the observer.But this appear to the observer as if the ray is reflected from the ground. Hence we feel the illusion of water being present on road which is virtual image of the sky.