For light diverging from a finite point source,

Explain the refraction of a plane wave from a denser to a rarer medium using Huygens' principle.

The speed of light in water is less than the speed of light in air,confirms the assumption of which theory of light?

What is the shape of the wavefront of light diverging from a point source?

Which of the following generates a 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 $\phi_c \neq \phi_f$.