In desert, one may see images
of distant trees during high temperature . Since in nature,
images of trees are formed only in water, therefore one
is tricked into believing that a tree is situated near some
water body and its reflection is being seen.
However, on reaching spot , it is found that the water
has vanished . This phenomenon is called mirage.
Mirage occurs due to total internal reflection of light.
When the sun is high in the sky, the sand gets heated first
and then the layers of air above it. The rays from the trees
travel from an optically denser air layer to a rarer layer
and hence bend away from the normal. This bending continues
and a stage is reached where the angle of incidence becomes
greater than the critical angle and total internal reflection
The totally reflected rays that reach the eyes appear to
come from a point on the ground where the image of the tree
is formed. Thus one sees an inverted image of the tree though
there is no water around.
If you are still unclear about the phenomenon, please
refer the animation