Explain how to get a new wavefront in time $\pi $ using Huygen's principle for plane wavefront.

The figure shows a surface $XY$ separating two transparent media, medium – $1$ and medium- $2$. The lines $ab$ and $cd$ represent wavefronts of a light wave traveling in medium- $1$ and incident on $XY$. The lines $ef$ and $gh$ represent wavefronts of the light wave in medium- $2$ after refraction.

The phases of the light wave at $c, d, e$ and $f$ are $\phi_c,\phi_d, \phi_e$ and $\phi_f$  respectively. It is given that $\phi_c \neq \phi_f.$

Wavefront of a wave has direction with wave motion

The idea of secondary wavelets for the propagation of a wave was first given by

A light beam is incident on a denser medium whose refractive index is $1.414$ at an angle of incidence $45^o$ . Find the ratio of width of refracted beam in a medium to the width of the incident beam in air