What is the minimum nonzero thickness of the film, so that destructive interference

Question

A mixture of red light (λvacuum = 661 nm) and green light (λvacuum = 551 nm) shines perpendicularly on a soap film (n = 1.41) that has air on either side. What is the minimum nonzero thickness of the film, so that destructive interference causes it to look red in reflected light?

wrong check mark (answer in nm)

Answer 1

The film would need to be 1/4 wavelength thick for destructive interference, so once th light has gone through, reflected off the far side, and come back, it would be 1/2 wavelength out of phase.

Since you're trying to get rid of the green light, 1/4 wavelength of that is 137.75 nm.