Why does a thin soap bubble swirl with rainbow colors when soapy water is clear?
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After you watchWhy does a thin soap bubble swirl with rainbow colors when soapy water is clear?
The short answer
Soap film colors come from interference between reflections from its two surfaces. Thickness changes their phase difference. At extremely small thickness, the reflected waves largely cancel, making a patch dark against a dark background.
The whole story
How it works
The front reflection undergoes a phase reversal. As the extra optical path in the film approaches zero, that reversal leaves the two reflections nearly opposite. The model uses an illustrative six-band spectrum with optical path 2nt and n=1.33.
What people get wrong
Thinner does not always mean more colorful. An extremely thin film can reflect much less visible light than a somewhat thicker colorful film.
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
Why does a thin soap bubble swirl with rainbow colors when soapy water is clear?
Soap film colors come from interference between reflections from its two surfaces. Thickness changes their phase difference. At extremely small thickness, the reflected waves largely cancel, making a patch dark against a dark background.
What is different in the hidden experiment?
Predict the fate of an already colorful bubble patch as its top becomes almost vanishingly thin. Drain it and watch reflected light diminish through cancellation.
What limit should I keep in mind?
Thinner does not always mean more colorful. An extremely thin film can reflect much less visible light than a somewhat thicker colorful film.
Talk about it
- Before running the hidden experiment, explain your prediction.
- Compare the final picture with your prediction. Which visible change supports your explanation?
- What did the experiment keep fixed?
For grown-ups
The front reflection undergoes a phase reversal. As the extra optical path in the film approaches zero, that reversal leaves the two reflections nearly opposite. The model uses an illustrative six-band spectrum with optical path 2nt and n=1.33. Thinner does not always mean more colorful. An extremely thin film can reflect much less visible light than a somewhat thicker colorful film.
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?
