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Light bouncing off two surfaces

Why Are Soap Bubbles Colourful?

Bubble colours come from light reflecting off both sides of the film and interfering.

1 Light hits thefilm2 It reflects offboth surfaces3 The reflectionsinterfere
No pigment anywhere. Only geometry.

  1. Step 1 of 5

    There is no colour in the soap.

    Paint Colour from pigmentBubble Colour from thickness

    Soapy water is colourless. The colours come from the film's thickness, not from anything dyed into it.

  2. Step 2 of 5

    The film has two surfaces.

    The bubble wall Outer surface, water,inner surface

    Light reflects off the outer surface and again off the inner one. Two reflections leave the bubble, very slightly out of step.

  3. Step 3 of 5

    They add or cancel.

    In step Colour is brightenedOut of stepColour cancels out

    Where the two reflected waves line up, that colour brightens. Where they are out of step, it cancels and disappears.

  4. Step 4 of 5

    Thickness decides which colour survives.

    Thicker filmreds and goldsThinner filmblues andviolets

    Each thickness cancels a different wavelength, so different parts of the bubble show different colours — and they shift as it drifts.

  5. Step 5 of 5

    Black means it is about to pop.

    1 Colours swirl2A black patchappears3 It pops

    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

Bubble colours come from light reflecting off both film surfaces and interfering, so the colour depends on thickness.

Try it yourself

Watch a still bubble. Colours drift downward as the film thins, and black appears at the top just before it pops.

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