(1) Mirage is an optical illusion where it appears that water has collected on the road at a distance place but when we get there, we don't find any water.
(2) During a hot summer day, air just above the road surface is very hot and the air at higher altitudes is cool.
(3) As a result density of air increases with height, hence refractive index of air increases with density.
(4) So, the cooler air at the top has greater refractive index than hotter air just above the road. 5) When the light from a tall object such as tree or from the sky passes through a medium just above the road, whose refractive index decreases.
(6) If we take any two layers of air, light incident an angle i, upper layer works as denser medium and lower layer works as rarer medium. So light refracts far away from the normal.
(7) Light rays passes through down layers, at particular layer i > c, light is completely reflected due to total internal reflection. (DE)
(8) The refracted ray reaches the observer and appears as if the ray is reflected from the ground.
(9) Hence we feel illusion of water being present on the road which is the virtual image of sky and an inverted image of tree on the road.
