You blow a bubble from clear, see-through soapy water — so what is hiding inside its skin?
Look… a shiny bubble, floating up and up. You made it from plain, clear, see-through water — no paint at all. And a bubble's skin is the thinnest little wall of water ever. Now watch… the skin is going to get thinner, and thinner, and thinner. What will happen? Ready to guess… will it stay clear, or will something surprising come out?
After you watchYou blow a bubble from clear, see-through soapy water — so what is hiding inside its skin?
The short answer
A clear soap bubble shows rainbow colors because its skin is an incredibly thin wall of water. As the wall gets thinner, light bouncing off its front and back lines up to make one color at a time, so rainbow bands appear — even though the water is clear with no paint in it.
Try this next
- What if you looked at an oily puddle after the rain? Go find a shiny puddle outside and look for the swirly colors. Guess first: will a puddle hide rainbows like a bubble does?
- What if a bubble could never get thinner? Tap the bubble to thin it, then watch the colors. Guess first: would the colors keep changing, or hold still, if the skin got stuck?
The whole story
How it works
A bubble's skin has a front and a back, and light bounces off both. When the wall is thick, the bounces jumble together and the skin just looks clear. As gravity drains the wall thinner, down to about the size of a wave of light, one color's two bounces line up at each thickness and that color glows. Because the wall keeps thinning, the winning color keeps changing, so the rainbow bands slide around.
What people get wrong
Little kids often think the colors are paint or sparkle stuff mixed into the soap. There is no paint at all — the very same clear water makes every color. The colors come from light bouncing inside a wall so thin you almost can't imagine it.
The catch
The rainbows are beautiful, but you can't keep them — the wall never stops draining and getting thinner. At the very top it gets so thin it turns dark, and a moment later the bubble pops.
Questions kids ask
Is there paint or glitter in the soap?
No. The soapy water is clear, with nothing colorful in it. The colors come from light bouncing inside the bubble's super-thin skin, not from any paint.
Why do the colors keep moving around?
The bubble's skin keeps getting thinner as the water drains down. Because the thickness decides which color shows, the colors keep changing and the bands swirl across the bubble.
Why does a bubble pop?
As the skin drains it gets thinnest at the top. When it gets too thin, that spot can't hold together anymore, so the bubble pops.
Talk about it
- There's no paint in a bubble — so where do you think the pretty colors come from?
- Why do you think the colors keep sliding around and never sit still?
- What do you notice happening right before a bubble pops?
For grown-ups
This is thin-film interference. Light reflects off both surfaces of the soap film; for each thickness, the wavelengths whose two reflections return in phase add together and dominate the reflected color while others cancel. As the film drains and thins, the favored wavelength shifts and the color bands slide. When the wall thins well below the size of a light wave, all colors are suppressed and that patch goes black just before the bubble pops.
Keep going
What else makes you wonder?
- Where else have you seen a rainbow without any paint — maybe in a puddle, or a shiny CD?
- If a thin skin makes rainbows, what other thin, shiny things might hide colors too?
- What do you think the colors look like right before a bubble pops?