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 means

Explain refraction of plane wave with a thin prism. 

Explain the refraction of a plane wavefront with a thin convex mirror. 

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