Frequently, while driving along
a highway on a clear, hot day, we see what appears to be
pools of water some distance ahead. However, as we approach
it, we find that the road surface that seemed wet before
is actually dry. On looking ahead , the wet patch appears
to have moved further away on the road.
When the sun goes high in the sky, the road gets heated
first and then the layers of air above it. Thus the layer
of air closest to road is hottest and optically least dense
whereas, the layers higher up are colder and optically more
dense. The rays from the sky travel from an optically denser
to a rarer medium 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 takes place.
The totally reflected rays that reach the eyes appear to
come from a point on the ground where the image of the sky
is formed. Thus one sees a reflection of sky giving an illusion
of water, though there is no water around.
If you are still unclear about the phenomenon, please
refer the animation