Soap film illuminated by bright white light Destructive interference where film is very thin. - In the top portion of the film, only reflection phase shifts are important since the path length difference across the film is very small (L  0). In the soap film, there is one  reflection phase shift. All wavelengths interfere destructively. - The film gets progressively thicker from top to bottom (due to gravity). Depending on the thickness, destructive interference at particular wavelengths (colors) takes place in the lower portions.