80. Soap Film

In this pretty demonstration light reflected from a soap film is focused on a rear projection screen. Gravity makes the soap film thicker at the bottom, so the image is a series of colored fringes produced by interference between the light reflected from the front and back surfaces of the film. As the water evaporates, the soap film becomes thinner, and a black area appears at the top of the film (bottom of the screen, if using a lens). When it becomes thinner than 1/4 (lambda/n), it represents destructive interference owing to the 180° phase change at the outside surface. The images below show this effect. (The phase changes at free and fixed ends can be demonstrated with the Bell Wave Machine.)