All objects reflect some of the colours and then our eyes are able to see object of different colours. But when we see mirage in a desert then it seems that the sand of the desert is acting as a mirror. So what makes the sand a mirror in this case? Is this because the light reflected from an object when falls on the sand is also reflected back to our eyes from the sand ? What makes the sand to behave like a mirror? I know about total internal reflection but then what makes the sand to reflect so clear image of an object?
Light travels very slightly faster in hot air than it does in cool air. In bright sunlight, the surface of the ground gets hotter than the air a few feet above it, which means that the air right next to the ground is hotter than it is a few feet above it. Therefore, a ray of light that is approaching the ground at a shallow angle will speed up a tiny amount as it gets close to the ground while the rest of the light ray that is a few feet above the ground does not.
This has the effect of gently bending the light beam up and away from the surface of the ground just as if it had struck a mirror lying on the ground.