Question

A beam of coherent light of wavelength $550\text{ nm}$ is incident normal to the plane of a pair of two slits $S_1$ and $S_2$ each of width $1.2 \times 10^{-6}\text{ m}$ separated by $1.1\text{ mm}$. Dark and bright fringes are observed on a screen $2.2\text{ m}$ away from the plane of the slits.
Calculate :
(I) fringe width.
(II) distance of the second dark fringe from the central maximum.
(III) what will happen when the entire apparatus is immersed in water.

Hint:

Remember that optical path lengths change when a system is placed in a medium. For any medium of refractive index $n$, the entire interference pattern scales down by a factor of $1/n$. Thus, fringe width, fringe positions, and angular widths all decrease by exactly $1/n$.

Answer 1