Question

Suppose Young's double-slit experiment is performed in air using red light and then the apparatus is immersed in water. What happens to the interference pattern on the screen?

Answer 1

Underwater, the wavelength of the light decreases according to $λ_{water} ​=λ_{air} ​/η_{water}$

​. Since the angles between positions of light and dark bands, being small, are approximately proportional to λ, the underwater fringe separation decrease.