In the given figure, a wavefront $AB$ moving in air is incident on a plane glass surface $XY$. Its position ${CD}$ after refraction through a glass slab is shown in figure. The ratio of width of wavefront $AB$ to the refracted wavefront $CD$ is,

Assertion: Contribution of the wavelets lying on back on a wavefront is zero.

Reason: The contribution of a wavelet in any direction making angle $\theta$ with the wavelet is proportional to $\frac{1}{2}(1+\cos \theta)$. In this case, $\theta =180°$.

The direction of wavefront of a wave with wave motion is,

Which one of the following phenomena is not explained by Huygens' construction of wavefront?

A parallel beam of monochromatic light is incident on a glass slab at an angle of incidence $45^{\circ} .$ Find the ratio of width of beam in air to that in glass (If refractive index of glass is $1.5$).

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