Physics Constructive Interference- Please Help

Question

A soap film (n=1.33) is 765 nm thick. White light strikes the film at normal incidence.What visible wavelengths will be constructively reflected if the film is surrounded by air on both sides?

Answer 1

I will add to the answer on my video. The problem is the same. EXCEPT the"With air on both sides" , the equation m*Lambda)= 2*n*d*cos(Theta) is constructive instead of destructive interference. The phase flipped twice. This time they told you to use the normal so cosine=1. but m can be more harmonics than just m=1.

m*Lambda= 2* n* d where d is the thoickness of the soap bubble. We don't need to know d

Lambda = 765 nm, 765nm /2, 765nm/3....= 765 nm, 382.5 nm, 255 nm etc which is twoward the UV spectral region