What is a mirage?
An invisible cloak.
No.
Mirage is an illusion in which it appears as if there is water present on the ground.
On a hot day, the land heats the layers of air closer to it.
However, as we move higher, the layers of air get relatively cooler.
Thus, the hot lower layers of air near the ground form rarer media.
Cold upper layers form denser media.
Now, when light coming from sky travels from denser to rarer media.
It bends more and more until it exceeds the critical angle.
i.e.the angle beyond which light gets reflected.
This phenomenon is called total internal reflection.
When this reflected light reaches our eyes, our eyes trace it backward as a straight line.
Hence, the reflection of blue sky appears on the ground.
Our brain thinks that there is water, thus causing a mirage.