Plate 28 · Light & Sound
Where a Bubble’s Colours Come From
Soapy water is colourless, yet blow it into a bubble and it shimmers like a rainbow, its colours forever flowing. They aren’t dyes. The light is being sorted by a film hundreds of times thinner than a hair.
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Two Reflections
A bubble’s film has two surfaces, an outer one and an inner one. Some of the light that hits it bounces off the outer surface; some goes into the film and bounces off the inner surface.
The two reflections travel almost the same way to your eye, except that the inner one goes a little further: across the film and back. One more twist: the reflection off the outer surface comes back flipped, crest turned to trough.
A bubble’s film, schematic · thickness enlarged thousands of times
Adding Up, or Cancelling Out
Light is a wave. If the crests of the two reflections line up, they reinforce each other, and that colour looks bright. If one’s crests meet the other’s troughs, they cancel, and that colour disappears.
Which colours add up and which cancel depends on how thick the film is. Take some colours out of white light, and what is left looks coloured.
Thickness Makes the Colour
As a film gets thicker, the colours appear in a fixed order: from black, through grey-white, then yellow, magenta, blue, then green, yellow, red, and around again, paler each time. Isaac Newton recorded this sequence more than three centuries ago.
Below are the colours worked out from the interference of light for films of different thickness. A difference of a few tens of nanometres can change the colour completely.
Film thickness and colour · computed from thin-film interference
A Flowing Rainbow
The water in a bubble’s film is always draining downwards, and slowly evaporating. So the top gets thinner and the bottom stays thicker, and the colours line up in bands from top to bottom, flowing and swirling all the time.
When the top thins to about 30 nanometres or less, the two reflections cancel almost completely and it turns black. See a black spot at the top of a bubble, and it won’t last much longer.
Colour From Structure
The same trick turns up elsewhere: oil on a puddle, the purple coating on a camera lens, the gleaming wings of some butterflies and the shells of some beetles. Their colours come not from pigments but from layers so thin they sort the light. This is called structural colour.
Your Turn
Set how thick the film is on average and watch the colours change. Let the water drain and watch the bands form, and the top slowly turn black.
Try thisThicken the film to 1,200 nm: what happens to its colours?
What the model leaves out
The colours are worked out for interference with light striking the film straight on; seen at a slant, light travels further through the film and the colours shift a little, which the picture roughly includes. A real bubble’s thickness is set by gravity, evaporation and surface tension together; here slowly flowing “noise” stands in for it. Screens can’t show every colour, so the most vivid ones are only approximate. The colours here are also much stronger than in a real bubble, which reflects only a few per cent of the light.
A common misreading
A bubble’s colours are not the colour of the soap, and they don’t come from splitting light the way a prism does. They come from two reflections strengthening or cancelling each other. That is why a film thin enough turns black: the outer reflection comes back flipped, so with almost no extra path the two meet crest to trough and cancel.
Sources
Follow the question
Plate 21Three Cones, a Million ColorsSunlight holds a smooth sweep of colors, but our eyes see it with just three kinds of cells. Every color you see is those three cells taking a vote.
Plate 07Why a Rainbow Is RoundIt does not hang anywhere in the sky. It is a ring of light 42 degrees around the shadow of your own head.
