Line perpendicular to wave front gives direction of which quantity? 

On the basis of Huygen's secondary wave principle explain refraction of light and derive Snell's law.

 A slit 4 cm wide is irradiated with microwaves of wavelength 2 cm. Find the angular spread of the central maximum assuming normal incidence to the plane of the slit.

A small transparent slab containing material of $\mathrm{\mu } =1.5$ is placed along ${\mathrm{AS}}_2$. What will be the distance from $O$ of the principal maxima and of the first minima on either side of the principal maxima obtained in the absence of the glass slab.

A beam of light consisting of two wavelengths, $650 \mathrm{nm}$ and $520 \mathrm{nm}$, is used to obtain interference fringes in a Young’s double-slit experiment. Find the distance of the third bright fringe on the screen from the central maximum for wavelength $650 \mathrm{nm}$.