Explain refraction of light on the basis of wave theory. Hence prove the laws of refraction.

 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.

Four identical monochromatic sources $\mathrm{A},\mathrm{B},\mathrm{C},\mathrm{D}$ as shown in the figure produce waves of the same wavelength $\lambda$ and are coherent. Two receiver $R_1$ and $R_2$ are at great but equal distances from $B$. Which of the two receivers can distinguish which of the sources $B$ or $D$ has been turned off?