Question

how does an mirage form?

Answer 1

It is because of the denser air below 
as light passes through from the less dense air to the dense air, refraction takes place and a mirage is formed.

Hope this helps:)

Answer 2

Mirages are like the optical illusions that are formed due to the total internal reflection. This happens when a light beem bends. For example, we see mirages like water ponds in a hot day in the desert. When light travels through the atmosphere from cooler to warmer it bends due to the change in the speed of light in the space and of the earth. This is what happens!! This is why we see these mirages like water in the roads when the light touches the hot air near the ground. Hope this helps!!