$A$ light beam travelling along the $X$-axis with a planar wavefront is incident on a medium of thickness $t$. In the region where light is falling,the refractive index varies such that $(dn/dy) > 0$. The light beam on the other side of the medium will emerge:

The wave front is a surface in which

Explain the reflection of a plane wavefront from a concave mirror.

By Huygen's wave theory of light,we cannot explain the phenomenon of

Assertion $A$: For light diverging from a point source,the intensity at the wavefront does not depend on the distance.
Reason $R$: In a diverging beam of light from a point source,a spherical wavefront is observed.

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 $\phi_c \neq \phi_f$.