If you’ve ever blown a bubble before, you’ve probably noticed the rainbow-like color you can often see on its outer layer. What causes these beautiful colors? A phenomenon known as iridescence.
What Is Iridescence?
Bubbles get their color due to a phenomenon known as iridescence. The bubble isn’t actually colored. It is distorting light.
A bubble’s shell is made up of a layer of water surrounded by two layers of soap film. When a ray of light hits the bubble, it does one of three things. The light will either:
- Reflect off the outer layer
- Enter the film and exit after bouncing off the inner layer
- Enter the film, and exit after bouncing between the two layers any number of times.
The colors you see is largely due to the thickness of the bubble. From the Wikipedia article on soap film:
Since each traversal of the film causes a phase shift proportional to the thickness of the film and inversely proportional to the wavelength, the result of the interference depends on these two quantities. So at a given thickness, interference is constructive for some wavelengths and destructive for others, so that white light impinging on the film is reflected with a hue that changes with thickness.
A change in colour can be observed while the bubble is thinning due to evaporation and draining. Thicker walls cancel out red (longer) wavelengths, causing a blue-green reflection. Later, thinner walls will cancel out yellow (leaving blue light), then green (leaving magenta), then blue (leaving a golden yellow). Finally, when the bubble’s wall becomes much thinner than the wavelength of visible light, all the waves in the visible region cancel each other out and no reflection is visible at all. When this state is observed, the wall is thinner than about 25 nanometers, and is probably about to pop. This phenomenon is very useful when making or manipulating bubbles as it gives an indication of the bubble’s fragility.
Iridescence can be found often in nature, from a butterfly’s wings, to the eyes of a cat.