Thin Film Interference Using Soap Bubbles

This demonstration shows the colors of light produced in a soap film held across a vertical ring. The open end of a coffee can provides a rigid ring that can be accurately relocated for projection after dipping in soap solution. As the fluid migrates downward, the differential thickening reinforces longer wavelengths lower in the soap film. This leads to horizontal bands of spectral color visible in the film. As the film thickens toward the base, the color bands move. Just before the bubble pops the top section of the film becomes so thin that it cancels all colors and it appears black.

To make this visible to a class, the white quartz light of a slide projector is reflected from the soap film to a short focal length condensing lens. This permits the color pattern to be projected onto the OHP screen in the room. It should be noted that the lens projection inverts the image, so all the patterns described above will be seen upside down.