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Why are these bubbles different colors?

Answer:

The different colors are the result of different thicknesses for the film of the bubble. When light hits the bubble, some of it is reflected, and some passes into the liquid film. When that light reaches the inside surface of the bubble, again, some is reflected and some passes through. The light that is reflected from the outside can be slightly out of step with the light reflected from the inside. How much it will be out of step depends on the thickness of the film.

Because the light waves are out of step, some of them are cancelled out. If one wave is up (crest) and the other wave is down (trough), then they cancel, and you don't see any of that color light.

The thinnest part of the bubble will look black, with all the light canceling. You see that just before the bubble pops. If the film is a little thicker, it will look yellow. A little thicker it will look magenta. A little more lets you see blue, and even more looks blue-green.

If the film is even thicker, then you go back to yellow again, and then magenta, blue, and back to blue-green, and so on.

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