MIRAGE. HOW IS IT FORMED ?

MIRAGE is formed by the phenomenon of total INTERNAL reflection of light. Mirage is formed in extremely high or low temperature conditions.
For example: When a sand surface (desert) or road surface is heated, the air above this surface is also heated and rises up.
As a consequence, the density of air in the lower level is less than that of upper level.
  Now , the light from any object starting from the upper layers (denser medium) travels to lower layers ( rarer medium), total internal reflection takes place i.e, the light is reflected back towards the denser medium. When these light rays reflected back, it seems that the light rays are refected from some bright reflecting surfaces on the ground.  Hence the  image is seen below the object.