Optics > Interference > Thin Films DCS# 6D30.20

SOAP FILM INTERFERENCE -- WHITE LIGHT



  APPARATUS
 
round wire frame 202-21-C4
bubble solution 202-21-C
250 ml beaker 101-01-C
slide projector 202-09-F
slide with circular aperture 202-09-C2
projection lens 101-17-E2


DESCRIPTION Reflect white light off of a soap film onto a screen to show an example of thin film interference.  Light waves reflected from the front and back surfaces of the wedge-shaped film interfere and produce horizontal bands of color.

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

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 λn/4 for any visible light, and it appears black.

NOTES

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.