Q: Find the thickness of a transparent plastic plate which will produce a change in optical path equal to the wavelength λ of normally. The refractive index of the plastic plate is μ .
Sol: When light travel a distance x in a medium of refractive index μ , its optical path = μ x
Change in optical path = μ x – x = (μ – 1) x.
This is to be equal to λ .
But , (μ – 1) x = λ
The thickness of the plate x = λ/(μ-1)