Interference between waves occurs whenever waves travel through the same space. Interference occurs when two rocks are tossed near each other into a lake. Circular waves on the surface of the water spread out from where each rock entered the water. Where the crests of two waves meet, interference between the waves causes the motion of the surface of the water to increase. Where a crest and a valley meet, interference reduces the motion of the water's surface.
Similar interference can occur in waves of light.
Waves of light reflected from the inner and outer surfaces of the film of a soap bubble can interfere with each other. Where the crests of the light waves reflected from the inner and outer surfaces of the film meet, the intensity of the light increases. If the crest of a wave reflected from the inner surface meets the valley of a wave from the outer surface, the intensity of the light will be diminished. Whether the crest of a wave meets another crest or a valley is determined by the length of the wave and by the thickness of the film. If the thickness of the film is a multiple of the wavelength of the light, the crests of waves reflected from the inner surface will meet the crests of waves reflected from the outer surfaces.