A mirage is an optical illusion that occurs due to atmospheric conditions by which reflected images of distant objects are seen. In other words, it’s a refraction phenomena in which the image of some objects appear displaced from its true position.
Mirages form when light rays emitted from a source or reflected off an object are bent, as the path of the light ray crosses air layers of different densities.
Common examples of a mirage are the appearance of water some distance down the highway on a hot summer day and seeing a lake in the desert.
Oasis or viewing a lake in the desert happens when light passes through two layers of air with different temperatures. The desert sun heats up the sand, which in turn heats up the air just above it. The hot air then bends light rays and reflects the sky. So when you see it from a distance, the different air masses colliding with each other act like a mirror. And the desert ahead seems to have become a lake which is actually a reflection of the sky above.
Mirages can be seen almost anywhere – those shimmering heat hazes that appear on the road ahead of you on sunny days, but disappear as you approach are mirages. Again, in the countryside you may seem to see a small lake or a pond near trees or in a field, which is also created by the same concept, i.e. differences in temperature between the ground temperature and the air just above the ground level.