There are many other experiments you can do to explore how light waves behave when they are reflected off soap film. We've included some of them in the video demonstration above.
For example, using the larger cards, try adding red wavelengths to the experiment. You can also adjust the thickness of the soap film by moving the masking tape lines closer together. Try soap films that are one blue wavelength apart, half a blue wavelength apart, a quarter of a blue wavelength apart, and a very thin soap film. To make the very thin soap film, you can use one strip of masking tape and draw a line down the center to delineate the front and back of the very thin soap film. Alternatively, you can use a single piece of string, as we do in the video.
When experimenting with the thickness of a soap film, always arrange the incoming wavelengths so that a maximum of each wave hits the front surface of the soap film.
To model how green light would behave, make green wavelengths out of 4 x 6-inch cards (roughly 102 x 152 mm; closest metric equivalent is A6) and conduct the experiment again.
Soap films that are very thin compared to the wavelengths of light moving through them reflect no light at all, making them invisible. You see this at the top of the soap film in the Soap Film on a Can activity. Moving down the film, when it is half of a wavelength of blue light thick, the blue waves add up out of phase and cancel. At this point, the soap film is now a quarter of a wavelength of red light thick and the red waves add up in phase, resulting in a reddish color band.
Blue light is canceled at soap-film thicknesses that are even multiples of one-half blue wavelength, and strengthened at odd multiples of a quarter blue wavelength. Red light is strengthened at multiples of a quarter red wavelength. The result is alternating bands of bluish and reddish light as the film grows thicker—like contour lines on a topographic map.
In reality, the wavelength of light actually changes when it enters the soap film—we have not added this additional complication to our model. To factor this in, think of the thickness of the soap film in terms of the number of wavelengths measured in the soap film. Since the index of refraction of soap is a little higher than water, for soap use n = 1.4. To calculate the wavelength of light in soap, divide the wavelength in air by 1.4.