Demos: 7B-11 Color in Thin Films
Interference between light reflected from the front and the back of a thin soap film produces colored bands with white light and dark bands with monochromatic light.
Directions: Darken the room. Dip the frame into the soap solution. Adjust the mirror so that the light beam is incident on the soap film. The projection lens should show the image on the distant screen. To show the dark bands, change to a monochromatic light source.
Suggestions for Presentation: Because the soap bubble is vertical, gravity will cause it to thin out from the top down. Eventually the film will become thin enough to show a dark region. However, the projection lens inverts the image so that it appears to darken from the bottom up. Point out this discrepancy to the students (or use it as a discussion point).
Applications: There are many instances of thin film interference in nature and in human-made systems.