The figure shows plane waves refracted from air to water using Huygens's principle. $a, b, c, d, e$ are lengths on the diagram. The refractive index of water with respect to air is the ratio:

$A$ point source of light is placed at the focus of a concave mirror. Consider only paraxial rays. The shapes of the wavefronts of incident and reflected lights respectively are

Initially,parallel cylindrical wavefronts travel in a medium with a refractive index $\mu(I) = \mu_0 + \mu_2 I$,where $\mu_0$ and $\mu_2$ are positive constants and $I$ is the intensity. The intensity decreases as the radius increases. What happens when it enters the second medium?

Explain the use of wavefront to understand wave propagation.

The phase difference between any two particles in a given wavefront is . . . . . . rad.

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 $\phi_c \neq \phi_f$.