Thin-film interference
by Chris Woodford. Last updated: August 2, 2023.
What do soap bubbles, peacocks, compact discs, and telescopes have in common? On the surface, nothing at all! Then again, if you look closely at the surface, in just the right kind of light, you'll see something you might not expect: unusual, often spectral colors. Twist a compact disc near a window, in the sunlight, and you'll get all kinds of neat spectral effects. You can do the same thing with soap bubbles: if you blow gently on their surface, rainbow colors will swim crazily in front of your eyes. Quite what you can do with a beautiful, iridescent peacock, I have no idea, but those feathers are surely worth more than a second glance? Inspect the surface of a decent telescope or pair of binoculars and you'll see a bluey-purplish color on the glass. So, what do all these colorful things have in common? They all use interference caused by thin films of materials or similar effects. How do they work? Let's take a closer look!
Photo: Spectral colors in simple soap bubbles. Watch how the colors change as you blow across the surface
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Contents
What turns white light into colored light?
We've all seen rainbows—but let's recap briefly on where their colors come from. Rainbows happen when droplets of water in the sky bend light beams coming from the Sun. Remember that ordinary visible light ("white light") is made from many different colors? Different colors of light are made by light waves of different wavelengths. When different wavelengths of light pass through a raindrop, they bend by different amounts, with the blue light bent more than the red. The bending effect, which is called light refraction, effectively splits one beam of light into the different colored rays of light from which it's made. So a huge cloud of raindrops will split a sunbeam and bend it into a spectrum, with violet and blue at one end and red at the other. That's how water droplets make colored light—but something slightly different is happening in soap bubbles.
Photo: Rainbow: The water droplets refract (bend) the shorter wavelengths of light (blue) more than the longer wavelengths (red), so blue is always on the inside of the curve.
What is interference?
If you've ever tossed pebbles in a very calm lake, you'll have spotted ripples spreading out in circles. If you throw two pebbles so they hit the water at the same time, a meter or two (a few feet) apart, you'll get two sets of ripples rippling toward each other. Where the ripples meet they overlap. Some of the ripples add together, some cancel out. This is called interference. When the ripples add, it's constructive interference; when they cancel out (or subtract), it's destructive interference. The result of interference is a brand new set of ripples quite different from the ones that either stone makes by itself. Interference can happen with light waves too—and it's essentially what's happening in colorful soap bubbles.
Artwork: Constructive interference makes waves bigger; destructive interference cancels them out. Adding waves together this way is called superposition. The waves on the left interfere constructively because they are in step (in phase); the ones on the right interfere destructively because they are out of step (completely out of phase or in anti-phase).
How does a soap bubble make colors?
To figure out why a soap bubble does funny things to light, we need to know a bit more about bubbles themselves. What are they anyway?
Soap is a kind of detergent and the bubbles it makes are a bit like balloons filled with air, but with one important difference. Where a balloon is made of fairly sturdy latex (thin rubber, in other words), the edge of a soap bubble is made from a thin film of soap and water. You make a soap bubble a bit like you make a sandwich. You need an ultra-thin layer of soap (like one piece of bread), then a layer of water (your filling, in the middle), and then another layer of soap (the top layer of bread). Wrap your sandwich into a perfect sphere and—hey presto—there's your soap bubble. How do you make a soap and water sandwich wrap into a sphere? Easy. Blow on soapy water! You'll find the soap-sandwich film wraps up all by itself, trapping the air inside. And what you get—if you're really lucky—is a perfectly spherical soap bubble held together by surface tension. It forms a sphere because that just happens to be the smallest, most stable structure it can have.
When white light shines on a bubble, strange things happen. Remember that light can behave like a wave. When light waves hit a bubble, some of them bounce straight back off the outer part of the soap film. Others carry on through but then bounce off the inner part of the film. So one set of light rays shine into a soap bubble, but two sets of rays come back out again. When they emerge, the waves that bounce off the inner film have traveled a tiny bit further than the waves that bounced off the outer film. So we have two sets of light waves that are now slightly out of step. Like two sets of ripples on a pond, these waves start merging. Just like on a pond, some add together and some cancel out. The overall effect is that some of the colors in the original white light disappear altogether, leaving other colors behind. These are the colors you see in soap bubbles.
Photo: In this soap-bubble closeup, you can see how the thickness of the soap film varies from place to place.
Stare at any one soap bubble and you'll notice that the colors vary across its surface (from place to place) and they also gradually change with time until the bubble bursts. Why is that? The soap film isn't quite the same thickness all over. Where the soap film is thick, red light is canceled out leaving the bubble looking blue or green. When the film is thinner, green is canceled, leaving the film magenta. If you blow on the film, the soap solution starts to evaporate and the bubble gets thinner. If you blow gently enough, you can make the colors change slowly from blue or green to yellow and violet, in the exact order you see them in a rainbow (red-orange-yellow-green-blue-indigo-violet). Try it next time you're in the bathtub or washing up at the sink! Eventually, as the film grows thinner and thinner, the colors disappear. The bubble goes totally clear. At this point, the film is just a few molecules thick. Then it bursts.
Artwork: Interference on the surface of a soap bubble: An incoming light ray is partly reflected by the top surface of the soap film and partly reflected by the bottom surface. The wave reflected from the bottom surface has traveled further (an extra distance equal to twice the thickness of the film) so emerges out of step with the top wave. When the two waves meet, they add together, and some colors are removed by destructive interference. Where the film is thickest, the bubble appears more blueish; where it's thinner, it will look more violet or magenta.
