Optics > Interference > Thin Films DCS# 6D30.20
SOAP FILM INTERFERENCE -- SODIUM LIGHT
APPARATUS
 
round wire frame 202-21-C4
bubble solution 202-21-C
250 ml beaker or plastic cup
 101-01-C
sodium lamp 101-08-A
ground glass screen
 113-01-A
video camera or document camera
 202-09-D
DESCRIPTION Use a camera to display the thin film interference pattern produced when a soap film is illuminated by sodium light.

Constructive interference occurs when thickness of film = (m+1/2)λo/2nfilm, so horizontal bright and dark fringes are produced by the wedge-shaped film.  As soap drips off the frame and the film thins out, the fringes can be seen moving down the film.  Just before it breaks, the top part of the film thins to less than λn/4, and appears black.

 NOTES The overhead lights should be turned off during this demonstration.

Use the ground glass screen to diffuse the light from the sodium lamp and uniformly illuminate the soap film.

Manually adjust the camera iris.

Soap solution recipe: 1 part Dawn, 1 part glycerine, 12 parts water.  If using commercial bubble solution, dilute it with an equal amount of water, otherwise a turbulent-looking pattern obscures the expected horizontal bright and dark bands.

Clip the metal disk to the wire frame to aid in aligning and focusing.

The photos below show the white light and sodium light versions set up on the same cart.  Here the document camera is used in place of the video camera.