Why soap bubbles swirl with colour, then turn black just before they pop
30seconds
Soap has no dye, yet bubbles shimmer with magenta, gold and green. The colour comes from thickness: light bounces off both the front and the back of the film, and depending on how far the second reflection travels, some colours cancel and the rest survive.
As the film drains, the bands slide downward and swirl. Just before it pops, the top turns black: the film is now only tens of nanometres thick, so thin that the two reflections almost cancel out. Isaac Newton described that black spot in 1704.
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