Young's double slit experiment is first done in air and then in a medium of refractive index $\mu$. If the 7 th dark fringe in the medium lies where the 4 th bright fringe is in air, then the value of $\mu$ is :

An EM wave of frequency $5 \times 10^9 \mathrm{~Hz}$ falls normally on a rectangular slit of width 3 cm . What is the total angular width of the central maxima?

In the Young's double slit experiment, when a monochromatic light is used, fringe width obtained is 1 mm . If the wave length is halved and the slit width is doubled, what will be the new fringe width?

A monochromatic light of wave length $6000^{\circ} \mathrm{A}$ is passed through two media A and B of thickness 10 cm and 16 cm respectively. The number of waves in $A$ is $\frac{1}{2}$ that of $B$. If the refractive index of $A$ is $\frac{4}{3}$, find the refractive index of $B$.