Subject: Applied Physics 2

Topic: Interference And Diffraction Of Light

Difficulty: Medium

A bright centre in reflected system can be obtained by selecting lens, medium and the glass plate in such a way that refractive index of lens is less than the medium and refractive index of medium is less than the plate. That means $μ_1$ < μ < $μ_ 2$.

So that there is phase reversal of П at two places ( at B and D) and total phase difference is zero.

In this case Δ=2μ t cos(r+ϴ) ±λ/2 ±λ/2.

i.e Δ=2μtcos(r+ϴ) ±λ

At the point of contact thickness is zero, therefore optical path difference becomes only which is condition for brightness. Therefor bright centre can be obtained.

If the light of red colour is used first and then a blue light, red colour would have a larger diameter as red colour have more wavelength than blue light.