In the adjoining diagram, a wavefront $AB$ moving in air is incident on a plane glass surface $XY$. Its position $CD$ after refraction through a glass slab is shown along with the normals drawn at $A$ and $D$. The refractive index of glass with respect to air $(\mu = 1)$ will be equal to

Huygens' principle of secondary wavelets may be used to

Given below are two statements:
Statement-$I$: $A$ plane wave after passing through a prism remains a plane wave,but passing through a small pinhole may become a spherical wave.
Statement-$II$: The curvature of a spherical wave emerging from a slit will increase for increasing slit width.
In the light of the above statements,choose the correct answer from the options given below.

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 wave has what direction with respect to the wave motion?

$A$ light wave travels in a vacuum along the $x$-axis. Which of the following may represent the wave front?