Light bouncing off two surfaces
Why Are Soap Bubbles Colourful?
Bubble colours come from light reflecting off both sides of the film and interfering.
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Step 1 of 5
There is no colour in the soap.
Soapy water is colourless. The colours come from the film's thickness, not from anything dyed into it.
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Step 2 of 5
The film has two surfaces.
The gap between them is under a thousandth of a millimetre, and that is plenty. Light reflects off the outer surface and again off the inner one. Two reflections leave the bubble, very slightly out of step.
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Step 3 of 5
They add or cancel.
Where the two reflected waves line up, that colour brightens. Where they are out of step, it cancels and disappears.
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Step 4 of 5
Thickness decides which colour survives.
Each thickness cancels a different wavelength, so different parts of the bubble show different colours — and they shift as it drifts.
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Step 5 of 5
Black means it is about to pop.
When the film thins below a wavelength of light, all reflections cancel and a black patch appears. Seconds later it bursts.
Oil films on wet roads shimmer for exactly the same reason — two surfaces, two reflections, interference.
The short version
No pigment is involved. Two reflections a thousandth of a millimetre apart cancel a different colour at each thickness — and when the film thins past one wavelength they all cancel, so a black patch means it is about to pop.
Try it yourself
Watch a still bubble. Colours drift downward as the film thins, and black appears at the top just before it pops.