This Week's Experiment - #252 Bubble Colors

a clear drinking glass
water
soap (I used Dawn dishwashing soap)
a drinking straw

Fill the glass about 1/4 of the way with water. Add a few drops of soap. Stir this with the straw to spread the soap around and then blow through the straw to make some bubbles. Examine the bubbles and notice their shapes. Stand with the glass of bubbles in your hand and with a strong light behind you. As you turn the glass, you should begin to see bright colors. Where two bubbles touch, they form a flat surface and these flat surfaces are where you will see the bright yellows, blues, reds and greens. What causes these colors?

To find out, move the straw around to pop most of the bubbles and start again. This time, as soon as you blow the bubbles, hold up the glass and look for the colors. Where are they? Watch for a few seconds and you will see them begin to appear. As you watch, the colors will become brighter and brighter. Why is that? What is changing as time passes?

The change is the thickness of the bubble. When you first blow the bubbles, the film of water and soap is fairly thick. As you watch, the water begins to drain out of the bubbles, flowing downwards into the glass. As the film of the bubble gets thinner and thinner, something wonderful begins to happen.

This is where things get a little more complex. Don't let this scare you into stopping, because it truly is amazing. If you have ever seen a rainbow, you know that white light is made up of many different colors. The rainbow separates the colors by bending each one at a different angle. With the bubble, bending the colors to separate them, we remove some colors, so that we see the others.

To understand how the colors are removed, we need to think of light as waves. One of the weird things about light is that sometimes it acts as if it is made up of tiny particles and sometimes it acts as it if is made up of waves. For now, we will think of it as acting like waves. If you have ever watched waves, this will be much easier. If you have not, then go take a bath. While you are getting clean, make waves in the tub and watch carefully what they do. You will notice that when a wave hits the side of the tub, it bounces back. Light does the same thing, and we call it reflecting. Part of the light hitting the surface of the bubble goes on through and part of it bounces back or reflects back towards you. Of the light that goes on into the bubble, more of it is reflected back when it hits the inside surface.

When the light that reflects from the outer surface mixes with the light reflecting from the inner surface, something so strange happens that it might seem like magic. The light that continued on to reflect from the inner surface had to travel a tiny bit farther (the thickness of the bubble film) than the light reflected from the outer surface. This means that the two sets of waves are slightly out of step. If this difference is just right, some of the waves will cancel each other out and that color will seem to be missing. Starting with white light, if the difference in the two sets of waves is enough to cancel out the red light, then you would see a blue-green color. If the color yellow is canceled, you would see blue, and so on.

The difference in the two sets of waves is controlled by the thickness of the bubble. As the water drains from the bubble, the film gets thinner. When it is thin enough, the two sets of reflected light reach the right level and you begin to see the colors. Different thicknesses give you different colors. With a round bubble, the colors seem to swirl, because the curved surface means you are looking through different thicknesses, depending on the angle. When two bubbles are connected they form the flat surface that we have been looking at. Since it is a flat surface, the thicknesses are more even and easier to see. The film is thicker at the bottom and thinner at the top, forming bands of color. You can even see difference in this thickness as water flows through the bubble, producing swirls of color.

This same process, called destructive interference, happens other places besides bubbles. It is what produces the iridescence seen in insect wings, the rainbows seen from a film of oil on wet asphalt, and the beautiful colors of the New Zealand paua shells that I had so much fun collecting last year.

Well, this is getting far too long, so I will stop here, but you don't have to. With a little research, you can find all sorts of wonders hiding in a soap bubble. I hope you all have a safe and happy holiday season.

These experiments are from Robert Krampf - The Happy Scientist

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