Constructive interference occurs when film thickness, d = (m+1/2)λ_o/2n_film.

When the frame is first dipped in the bubble solution, d >> λ, and the reflected light appears white, since the condition for constructive interference is met for many wavelengths of visible light.  As soap drips off the frame and the film thins out, horizontal interference fringes can be seen moving down the film. Just before it breaks, the film thickness at the top becomes less than λ_n/4 for any visible light, and it appears black.

The projector is on separate support because otherwise the fan makes the cart vibrate and disturbs the soap film. 

The overhead lights should be turned off during this demonstration.

Projected pattern will be upside-down.

Soap solution recipe: 1 part dawn, 1 part water, 1 part glycerine.

Clip metal disk to wire frame for aligning and focusing.