Is Huygens' principle valid for longitudinal sound waves?

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 $\phi_c \neq \phi_f$.

The wavefront of a distant source of unknown shape is approximately

Explain the refraction of a plane wave by a thin prism.

$A$ wavefront is a surface

State and explain Huygens' principle.