Constructive interference in thin films (when the film material has a higher index of refraction):
Refractive index of water, η = 1.33
d = 390 nm (10-9m/1nm) = 390 × 10-9 m
Solving for the wavelength, λ:
A soap bubble is essentially a thin film of water surrounded by air. The colors you see in soap bubbles are produced by interference.
(a) What visible wavelengths of light are strongly reflected from a 390 nm -thick soap bubble?
(b) What color would a 390 nm thick soap bubble appear to be (green, purple, or yellow)?