Mirage is an optical illusion occuring in deserts and on roads on a hot summer day due to total internal reflection. Because optical density at different layers of air increases with height. As a result, light from tall object such as a tree passes through a medium whose refractive index decreases towards the ground. Thus, a ray of from such object successively bends away from the normal and undergoes total internal reflection, if the angle of incidence for the air near the ground exceeds the critical angle.