Find the thickness of a plate which will produce a change in optical path equal to half the wavelength `lamda` of the light passing through it normally. The refractive index of the plate is `mu`.

Explanation

Optical Path : If light travels a distance .x. in a medium of refractive index `mu`, the equivalent path in vacuum, it would travel in the same time is `mux`, which is called as optical path.
Change in optical path = `mux-x=(mu-1)x`
According to problem `(mu-1)x=lamda`
Hence, `x=lamda/(mu-1)` = thickness of plate.