# Bubble Color

From: DonRath@aol.com
Date: Tue Oct 27 1998 - 20:51:06 PST

I'm sorry that this answer is so long in coming, but I've been away for much
of the past month-and-a-half, and your question got forwarded to me while I
was away.

There are really a couple of different parts to your question. When you asked
how green soap can have bubbles that are clear, one thing to realize is that
bubbles are very thin, and you aren't looking through very much soap. The
thinner the layer of soap you look through, the lighter the color. So part of
the answer is that you might expect a bubble from green soap to look very
light green, or even colorless. You can see the idea here by pouring some cola
or orange soda (or even some food coloring in water) into a glass so that
there is a very thin layer over the bottom of the glass -- about an eight of
an inch or so -- and then looking down into the glass and through the bottom
at a piece of white paper. Then fill the the glass until it is about a
quarter to half full. Notice that the color gets much darker.

But that isn't the whole story, because the soap bubble from green soap
doesn't really look pale green or colorless in ordinary white light (room
light from incandescent or fluorescent bulbs, or sunlight). It has a whole
variety of colors that constantly change. So the other part of the story is
that as light hits the soap bubble, part of it reflects from the front surface
of the bubble, and part goes into the thin bubble and reflects from the back
surface. When both parts of the light get back into the air and travel to your
eye, they interact with each other to either partially reinforce or partially
cancel each other, depending on the color of the light and the thickness of
the part of the soap bubble they reflect from. Since the thickness of a soap
bubble varies from place to place on the bubble, and white light contains all
colors of light, one area of the soap bubble may reinforce red, but cancel
green and blue, while another may reinforce green, but cancel red and blue.
Thus different areas of the soap bubble have different colors.

The paragraph above has been an attempt to compress a whole lot of physics
into a very little space, so I'm not sure how clear it will be to you. I hope
it helps, but if you have more questions, let me know. Again, sorry for the
delay.

Don Rathjen
donr@exploratorium.edu
OR
donrath@aol.com

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