You coat a glass lens (n = 1.65)with MgF₂ (n = 1.38)to reduce reflection.The minimum thickness of coating required to produce destructive interference in reflected light whose wavelength in air is 560 nm is A)102 nm B)140 nm C)203 nm D)280 nm E)None of these is correct.

Question

Quizplus · Accepted Answer

For destructive interference, the optical path difference should be (m + 0.5)λ/n, where m is an integer and n is the refractive index of the coating. For minimum thickness (m=0), the thickness t = λ/(4n) = 560 nm / (4 * 1.38) = 101.45 nm, approximately 102 nm.

You Coat a Glass Lens (N = 1