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Soap Bubbles and Butterflies

If you have some dish soap, a mug, and a bright light, we have a science experiment for you to try.

An iridescent soap bubble

We can think of light as waves traveling through space, like ripples across a pond. Light can have different wavelengths i.e. different distances between the crest of one wave and the crest of the next.

Different wavelengths of light give us the sensation of different colors. Those are some abstract, but basic ideas about light. Here’s how to see those ideas in action.

The Experiment

Make a solution of soapy water in a shallow bowl. Take a coffee cup and lower it, mouth down, into the soapy water, then lift it out gently. You want a thin soap-bubble film across the mouth of the cup (we explained why you need that thin film to create bubbles in this post, if you’re interested in how bubbles are made).

Now, hold the cup so the soap-bubble film is vertical. Hold it, so you can see the reflection of a bright light, such as the sky, in the bubble’s surface. After a few seconds you’ll see bands of color appear near the top of the bubble, slowly moving downward.

The soapy water has no color of its own. The thickness of the bubble makes the colors.

Wherever the bubble is thick enough to accommodate a whole number of waves of, say, red light, red light will be reflected to your eye, because red-light waves bouncing off the back side of the bubble will be exactly in step with red-light waves bouncing off the front of the bubble. The same goes for other colors. As the bubble’s thickness changes, so do the colors.

Iridescence

This is the same process that makes colors in oil floating on water. And most of the blue colors in bird feathers and butterfly wings also come about in the same way; Not through colored dyes (although scientists are attempting to create dyes to mimic this kind of coloring), but through reflection of light from both sides of a thin film.

  • C.J. Lynde, Science Experiences with Ten-Cent Store Equipment, 2nd ed. (1951);
  • R.P. Feynman, QED: the strange theory of light and matter (1985).

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