Why does a thin soap bubble swirl with rainbow colors when soapy water is clear?
After you watchWhy does a thin soap bubble swirl with rainbow colors when soapy water is clear?
The short answer
A soap bubble has colors because its wall is an incredibly thin sheet of water, and light bounces off both the front and the back of that sheet. The two bounces overlap as waves. At each tiny thickness one color's two bounces line up and add together while others cancel, so that color shows. The colors come from the wall's thickness, not from any dye in the soap.
Try this next
- What if you turned the bubble sideways or upside down instead of letting it hang? In the explainer, watch which part of the wall thins first as it drains. Predict where the rainbow bands and the black patch will appear if gravity pulled the water a different way.
- What if the wall could never get thinner than one wave of light? Drain the film and watch the moment the top goes black. Predict first: would the colors keep swirling forever, or stop, if the wall got stuck at one thickness?
The whole story
How it works
A bubble's wall has two surfaces, a front and a back, and light reflects off both. The back reflection travels a little farther because it crosses the wall twice. When the wall is much thicker than a wave of light, the two reflections jumble together with no single color winning, so the film looks clear. As the wall drains and thins to about the size of a light wave, at each thickness one color's front and back bounces come back in step and add into a bright band while other colors cancel. Because the wall keeps thinning, the winning color keeps changing, which is why the bands swirl and slide.
What people get wrong
Many people assume a bubble's colors come from dye or dirt in the soap, like the colors of paint. There is no pigment at all. The exact same clear, colorless soapy water makes every color simply by being a wall thin enough for light's two reflections to line up or cancel. The color is made by the light and the wall's thickness, not by any coloring in the water.
The catch
The colors are real and beautiful, but they are not painted on, so you cannot choose them, the wall's thickness at each spot decides which color appears. And the colors never hold still, because gravity keeps draining the wall thinner, the winning color keeps changing and the bands keep sliding. Just before a bubble pops the top turns black, where the wall has thinned below the size of any light wave, no color can line up, so almost nothing reflects.
Questions kids ask
Is there paint or dye in the soap that makes the colors?
No. Soapy water is clear and colorless and there is no pigment in a bubble. The colors are made by light bouncing off the front and back of the thin wall and the two bounces lining up or cancelling, so the color depends only on how thick the wall is at each spot.
Why do the colors keep swirling and changing?
Gravity pulls the water in the wall downward, so the wall is always getting thinner and the thickness is different at different spots. Since the thickness decides which color lines up, the winning color keeps changing, which makes the bands of color drift and swirl across the bubble.
Why does the top of a bubble turn black right before it pops?
As the wall drains it gets thinnest at the top. When it becomes much thinner than a single wave of light, no color's two bounces can line up, so almost no light is reflected and that part looks black. That black patch is a sign the bubble is about to pop.
Where else can I see these colors?
Anywhere a super-thin clear layer sits on something, like a film of oil or gasoline on a wet road, the soapy sheen on a CD case, or an oil slick on a puddle. They all show shifting rainbow colors for the same reason: light bouncing off the top and bottom of a thin layer and lining up at different thicknesses.
Talk about it
- There's no paint in a bubble at all — so where do you think the colors actually come from? Guess before you say.
- Why do you think the colors never sit still and keep sliding around?
- Right before a bubble pops, the top turns black. What do you think is happening to the wall there?
For grown-ups
This is thin-film interference. Light reflects from both surfaces of the soap film; the back reflection travels an extra path of roughly twice the film thickness, and the front reflection also gains a half-wavelength phase flip on reflecting off the optically denser film. For a given thickness, wavelengths that return in phase interfere constructively and dominate the reflected color while others cancel. As the film drains and thins, the favored wavelength shifts, so the color bands slide. When the wall thins well below a quarter of a wavelength, only the half-wave flip remains, all visible light is suppressed, and the film appears black just before it pops.
Keep going
What else makes you wonder?
- If a bubble's color comes from how thick its wall is, could you read the wall's thickness just by looking at the color?
- Soap bubbles, oil puddles, and a CD all show shifting rainbows — what else around you might be hiding a wall this thin?
- The top of a bubble goes black right before it pops — what would you see if you could watch the whole wall thin out in slow motion?