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Saturday Science: Brilliant Bubbles

Okay, so bubbles are pretty cool, right? There’s a reason people have been buying bubbles for bubble blowing for, like, billions of years. Okay, maybe not that long, but I like alliteration. And it’s not like you need to buy your bubbles. If you’ve ever taken a bath, you know that all you need is a little soap in your water, and BOOM! Bubbles, baby!

(If you haven’t ever taken a bath, try it. It can be quite relaxing. And also you probably smell really bad, you gross person.)

Something you may have noticed about bubbles at one time or another is that when the light hits them just right, they burst into a brilliant blast of color. In this Saturday Science, you’ll construct a sort of bubble observatory to make that happen on command, and we’ll learn just why bubbles can be so colorful.

Materials:

  • A small clear plastic lid (like from a yogurt container or something like it)
  • Clear Scotch tape
  • A flashlight
  • A spoon
  • A straw
  • Water
  • Dish soap
  • A 1/3 cup measuring cup
  • A big bucket or saucepan
  • Darkness

Process:

  1. First, you want to make your bubble solution.
  2. Put about a gallon of water into your bucket/saucepan.
  3. Mix in 2/3 of a cup of dish soap.
  4. (optional) Wait overnight. It’ll work fine immediately, but will work better if you wait.
  5. Now to start building your observatory. First, tape the plastic lid over the lighty-uppy end of the flashlight.
  6. Dip your finger in your bubble solution and wet the top of the lid. This’ll make it easier for bubbles to form.
  7. Pour a spoonful of bubble solution into the lid.
  8. Stick one end of your straw into the bubble solution in the lid and gently blow until you have a big bubble dome that covers the entire lid.
  9. Find your darkness (Closet? Bedroom with blinds drawn and lights off? Cave deep under Peru?) and turn on the flashlight. Hold it up so the bubble is just at eye level.
  10. Look at those colors! What are they doing? How are they moving? What colors do you see?
  11. Play with the bubble a bit. Try gently shaking it around or blowing on it. You can even stick the wet end of the straw inside the bubble without bursting it and blow gently inside. What happens then?

Summary

So how come all these beautiful bubbles have colors all up in ‘em? Well, the first hint is that you needed to have a flashlight for this to work: it’s all about the light. Specifically, how the light moves through the bubble. To really understand it we have to break it down into two different concepts: what light is and what bubbles are.

Let’s start with light. 

Light, as you may know, is a wave. Like an ocean wave, it travels in a certain direction and as it moves in that direction it also moves up and down, creating peaks (the highest point in the wave ) and troughs (the lowest point in the wave). Now, more than one wave can occupy the same space at the same time. When this happens, they interfere with each other. If two waves are traveling with each other and their peaks and troughs are aligned, they will add together, creating new peaks and troughs that are twice as far apart. This kind of interference is called constructive interference. The opposite can happen, too. If two waves are traveling and one is pushing up while the other is pushing down, the two waves will cancel each other out. This is called destructive interference.

Also, the length of a light wave (its “wavelength”) determines what color it is. Violet is the shortest wavelength we can see; red is the longest. When you have a roughly equal mix of all of the colors/wavelengths, you get white light. That’s right: white light is actually all of the colors of the rainbow. Incidentally, that’s why it can turn into a rainbow: the rain breaks the colors apart.

Onto bubbles. 

What is a bubble? Well, it’s a soap and water sandwich. Really. The thin layer of liquid that makes up the bubble (the “film”) is basically a layer of water molecules between two layers of soap molecules. It is very, very thin. 

In fact, it is so thin that it’s roughly only as thick as the wavelengths of visible light. Which means it can sorta bounce the light around inside it, and when the light waves meet, they interfere with each other. Some interference is constructive, making certain colors brighter. Other interference is destructive, fading some colors out. The result is the rainbow you see in a bubble’s film.

So why does it change? Well, the exact pattern of interference is determined by the exact thickness of the film, and if you move it or blow on it, the thickness shifts around, causing different colors to get brightened/faded at different places on the bubble. When the film is as thin as it can possibly be, right before the bubble pops, it’ll look black, because at that thickness, all of the different wavelengths get canceled out and there’s no color left to see.

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Categories: Science
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