A Soap Film Turns Black Where It's Thinnest

Feed the Reader

18 hours ago

4 min read

Dip a loop of string into some soapy water and lift it out, and you'll have a little window of soap film. Hold it up to the light and look at it.

There's colour. Bands of it. Purples, greens, golds, sliding slowly downwards like a very relaxed rainbow. And then, if you wait, something odd happens near the top. A patch appears that seems to have no colour at all. It looks black, almost like a hole has opened up.

It hasn't. The film is still there. In fact, that black patch is telling you something very precise about it.

Two reflections

To see what's going on, think about what happens to light when it hits a soap film.

A soap film has two surfaces: a front and a back. When light arrives, some of it bounces straight off the front surface. Some of it passes into the film, travels across to the back surface, and bounces off that instead. So there are two reflections heading back towards your eye, one from each side of the film.

But they haven't travelled the same distance. The one from the back has had to go across the film and back again, so it's slightly behind.

Light travels as a wave. And when two waves overlap, they can either add together or cancel out, depending on how they line up. If the peaks of one line up with the peaks of the other, they reinforce each other and that colour looks bright. If the peaks of one line up with the troughs of the other, they cancel and that colour disappears.

This is called interference. And whether a particular colour adds up or cancels out depends on how far the second reflection has to travel, which means it depends on how thick the film is.

Different colours of light have different wavelengths. So at a given thickness, some colours reinforce and others cancel. What you see is whichever colours survive. Change the thickness and you change the colour.

A map of thickness

That's why the colours come in bands. Each colour you see corresponds to a particular thickness of film. So the pattern on a soap film is, in effect, a contour map of how thick it is from place to place.

And the bands move because the thickness keeps changing. Gravity pulls the water in the film downwards. The liquid slowly drains towards the bottom, so the film gets thinner at the top first. As it thins, each part of the film works its way through the sequence of colours, and the bands appear to slide down.

You're watching a soap film drain, in colour.

The black at the top

Here's where it gets really interesting.

As the top of the film gets thinner and thinner, eventually it becomes so thin that the extra trip across the film and back is tiny. You might expect the two reflections to line up nicely and add together. After all, if the second reflection is barely behind the first, surely they're in step?

But they're not, and the reason is a quirk of reflection. When light bounces off the front of the soap film, going from air into the soapy water, it gets flipped: the wave is turned upside down. The reflection from the back of the film, going from soapy water back out into air, doesn't get flipped.

So at the very thinnest, the two reflections have travelled almost the same distance, but one of them is upside down. Its peaks line up with the other's troughs. They cancel. And because the extra distance is now so tiny compared with the wavelength of any visible light, they cancel for every colour at once.

No colour gets reflected. So the film looks black.

That's the counterintuitive part. The black patch isn't where the film has gone. It's where the film is thinnest. And if you watch a soap bubble carefully, you'll find that it goes dark at the spot where it eventually breaks.

Oil and lenses

The same physics turns up elsewhere.

The rainbow sheen you see on a puddle with a film of oil on it is thin-film interference too. The oil sits on top of the water, and reflections from its two surfaces interfere in much the same way.

And it's put to deliberate use. Anti-reflective coatings on lenses are thin films designed to make reflections cancel out. A coating about a quarter of a wavelength thick, with just the right optical properties, can make the light reflected from its two surfaces cancel, so that less light bounces off the lens and more of it passes through.

Try it at the sink

You can watch all of this for yourself with some water, a squirt of washing-up liquid and a little glycerine. Thread a piece of cotton string through two drinking straws to make a frame, dip it in the mixture and lift it out carefully. Hold it upright and watch.

You should see the bands of colour form, slide downwards, and eventually the black area appear at the top and slowly move downwards as the film drains.

Soap films are fragile things, though, so don't be surprised if yours pops before you've seen the whole show.

A ruler for the very thin

What's lovely about soap films is what the colours really are. They're not decoration. They're a measurement.

A soap film is far too thin to see its thickness directly. But the colours tell you anyway. Each band marks out a particular thickness, and the black at the top marks the thinnest of all. The rainbow is a ruler, for something too thin to measure by eye.