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                        takes place.
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.
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regards
Ramesh