Soap Bubble Colors
Destructive interference: 2nt = m l
Constructive interference: 2nt = (m + 1/2) l
For a given thickness of the soap film, some colors interfere constructively and others destructively.
Notes:
For example, let’s suppose the thickness of the film is about 3/16 of the
wavelength of red light (800 nm). Then, since the index of refraction of the
water is about n = 1.33 =4/3, 2nt = 400 nm, which means the red light reflected
from the inner and outer surfaces of the bubble will interfere constructively.
But, since this 400 nm is also the wavelength of blue light, the two
reflections will interfere destructively in blue.
Therefore, if white light is reflected from a bubble film that is 400 nm
thick, it will appear to have a red color.