A soap film is a soapy-water sandwich, with two outside layers of soap molecules forming boundaries around a layer of soapy water. The thickness of the soap film changes as the water drains down the inside of the film.
When light strikes the front surface of the soap film, some of the light is reflected (about 4%). The remainder of the wave is transmitted through to the rear surface. At the rear surface of the soap film, more of the light is reflected back to your eyes. The light reflecting from the front of the film meets up with the light reflecting from the back of the film, and the waves combine.
The beautiful colors you see on the soap film are due to interference patterns, created when light reflects off the two surfaces of the thin soap film. Interference patterns are created when two reflected waves line up in phase or out of phase. If two waves line up in phase, with crests together and troughs together, we say that the waves are interfering constructively. When two waves line up out of phase, crest to trough, we say that the waves are interfering destructively.
White light can be considered a mixture of three additive primary colors: red, green, and blue. If the thickness of the soap film is just right to cause the destructive interference of one of the additive primaries, you will perceive a mixture of the two remaining colors:
- white – red = blue + green = cyan (bluish green)
- white – green = red + blue = magenta (reddish blue)
- white – blue = red + green = yellow
Therefore, everywhere you see yellow, the film is just the right thickness to destructively interfere with the blue light waves, removing them. Where you see cyan, the red light has been removed. And where you see magenta, the green light has been removed.
The surface tension that allows a soap film to form is caused by the tendency of water to minimize its surface area. When a finger, pencil, or piece of PVC pipe is coated with bubble solution and inserted into the soap film, the film stays in contact with a like solution and remains continuous. A dry finger, however, interrupts that continuity, making it impossible to maintain surface tension, and the bubble bursts.