An optical illusion v/hich occurs mainly in deserts during hot summer is based on the principle of .

A mirage is an optical illusion common in deserts during hot summers, primarily caused by the bending of light through air layers of varying temperatures. The sun heats the ground, which in turn heats the air immediately above it, creating a temperature gradient where hot, less dense air lies beneath cooler, denser air. This gradient causes the refractive index of the air to change gradually. As light from the sky or distant objects travels toward the ground, it passes from denser to rarer air layers and undergoes continuous refraction, bending away from the normal. When the angle of incidence exceeds the critical angle, total internal reflection occurs, causing the light to bend upward toward the observer's eye. The observer's brain perceives this light as coming from a straight line, creating the illusion of a reflection on a water surface.