# Soap bubble

1. Apr 29, 2008

### salsabel

Which of the following colours would you expect to see near the bottom of a soap bubble, red or blue?

this is my solution
If the thickness of the soap bubble is much smaller than the wavelength of light, the result is destructive interference (dark color). The thickness required to produce constructive interference for red is larger than that required for blue.
therefore we see red at the bottom

is that right?

2. Apr 29, 2008

### Chi Meson

Your logic is ok, but the question is flawed. You are correct in your first statement about the "dark band" seen at the top of the bubble, and that red would be experiencing constructive interference at a thicker location than for blue, but...

Colors on soap bubble will cycle through the range of colors several times as the thickness increases . There is no "final color" at the bottom. The thickest part of a soap bubble is never defined to be either a multiple of the wavelength of blue OR red.

And in the real world, as you get to the bottom of the soap film, the colored bands get closer and closer until you can no longer determine a colored band at all. (I have 3 kids, I have been staring a bubbles a lot lately) Thin-film interference goes away when you can no longer see these bands.

Another "real world" note, the observed colors in a soap bubble are primarily due to the color that suffers destructive interference. Notice next time the colors you see are mostly yellow, cyan and magenta.