On a hot summer night, the refractive index of air is smallest near the ground and increases with height from the ground. When a light beam is directed horizontally, the Huygens` principal leads us to conclude that as it travels, the light beam:

Options

A. becomes narrower
B. goes horizontally without any deflection
C. bends downwards
D. bends upwards

Answer

D

Explanation

Consider a plane wave front travelling horizontally. As it moves, its different parts move with different speeds. So, its shape will change as shown in Fig.

`:.` Light bends upwards.