This five-foot-square soap film, created by pulling a cord, is a soapy-water sandwich, with two outside layers of soap molecules around an inner layer of soapy water. The thickness of the soap film changes as the water drains down the inside of the film.
When white light (made up of many colors) shines on the soap film, some of the light is reflected and the rest 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 overlap.
It’s this overlapping of the light waves that creates the beautiful interference colors you see on the soap film. Interference colors are created as two reflected waves overlap and line up either in phase or out of phase. If two waves line up in phase—that is, 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.
Constructive interference
Destructive interference
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 these additive primaries, you 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
In other words, 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.
When all the colors disappear, watch out: the soap film has gotten so thin that it’s about to pop.
You can find interference colors in all sorts of places—including butterfly wings, opal, oil slicks, and peacock feathers. (click image to enlarge)
If you wet your hand in the soapy water, you may be able to put your fingers through the film. You can also use a plastic ring to put a porthole in the bubble wall.
The surface tension that allows a soap film to form is caused by the tendency of water to minimize its surface area. Soap reduces surface tension by separating water molecules from each other. 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 the object without disruption. But a dry finger interrupts the continuity of the reduced surface tension “membrane,” causing the bubble to burst.
