A circular ring is dipped in a soap solution, creating a soap film, and held vertically. White light shines on the soap film. The reflected light consists of a number of horizontal colored bands.

Why do we get horizontal colored bands?

As you move down the film thickness gradually increases, and the thickness determines which color experiences constructive interference.

Watch one band of a particular color. As time passes the band moves vertically. Does it move up or does it move down? Why?

As time passes the liquid gradually moves down, so the film steadily gets thinner. To produce reflected light (due to constructive interference) of a particular color requires a particular thickness...over time that part of the film of the required thickness is further down the film, so the band drifts down.

If you watch the top of the film it gradually becomes white, and then it looks dark (non-reflecting) before the film pops. Why does that happen?

The film goes white when the film is too thin to give constructive interference for any color, but it's reflecting all colors partially, producing white light. To understand why it then goes black (non-reflecting) look at the first few steps in the seven-step method.

Step 1. Because the soap film has a higher index of refraction than air, the wave reflecting off the front of the film is shifted by half a wavelength.

Dt

=

ln 2

Step 2. Because the soap film has a higher index of refraction than air, the wave reflecting off the back surface of the film does not have a half-wavelength shift, but it does travel the extra distance of 2t.

D_b = 2t

Step 3. The relative shift is thus:

D

=

Db - Dt

=

2t -

ln 2

As t goes to zero the relative shift approaches half a wavelength, so when the film is very thin we get destructive interference. This means no light is reflecting from the film.