The occurrence of mirage can be explained as follows.
The air on the road surface consists of many layers. On a hot day, the air near the ground has a low specific heat capacity, hence the temperature increase faster.
The hot air becomes less dense than the cold air higher up.
A ray of light originated from the sky is refracted away from the normal as the light is travel from denser to less dense air.
As the air passes through the lower layers, the angle of incidence increases and the refracted ray is getting further away from the normal.
Finally, at a layer of air close to the road surface, the angle incidence exceeds the critical angle. Total internal occurs and the light ray bends upward towards the eye of the observer.
The observer sees the image of the sky and the clouds on the surface of the road as a pool of water.
Rain Bow
The spectrum of a rainbow is caused by total internal reflection in the water droplets.
Different angles of total internal reflection produces different colours.
