Soap Film on a Can

1. To make the soap solution, mix 1 part dishwashing liquid in 10 parts water. Tip: Soap films work best and last longer if you let them sit overnight, allowing the soap molecules to diffuse through the entire solution.
2. Pour the soap solution into the shallow dish, filling it to approximately 1/2 inch (1 centimeter) deep. 
3. Remove the lid from the film can.
4. Place the white paper on a tabletop. The white paper is important because light will then scatter up off the paper, bouncing off the soap film and into your eye.

Dip the open mouth of the film can straight down into the soap solution. When you pull it out, a soap film should have formed over the opening of the can.

Rotate the can so that the soap film is in a vertical plane. Hold it over the white paper in a brightly lit place.

You will see colors form and move around on the soap film. Over time, you might notice horizontal bands of color forming.

After a while, the top of the soap film becomes invisible because the soap film becomes thinner than a wavelength of light—under 300 nanometers thick. Even though you can’t see it, the soap film is still there! You can test this by poking a pencil point into the invisible region of the soap film—it will break.

Think of the soap film as a water sandwich: a layer of water held between two layers of soap molecules. When the soap film is vertical, gravity pulls the water down, causing the top of the film to become thinner and the bottom to become thicker.

The reflections caused by these different thicknesses of soap film then cancel out different colors, producing bands of color that stretch across the film can.

Light reflects from both the front and back of the soap film. The light waves reflected from the front of the soap film are inverted while those from the back are not. The two reflections combine, producing interference of light.

When interference of light occurs, some color wavelengths add up “out of phase”—the highest point of one wave lines up with the lowest point of the other—and are therefore canceled. Others add up “in phase”—where the highest point of one wave lines up with the highest point of the other—and are therefore strengthened.

Understanding the colors of the soap film from top to bottom:

When soap films are thin compared to the wavelengths of light moving through them, they reflect no light at all, making them invisible. You can see this happening at the top of your soap film. Poking the invisible soap film with a pencil point reveals its presence because the entire soap film breaks.

When the soap film is a quarter of a wavelength of blue light thick, blue light is reflected strongly. At this same point, the film is about an eighth of a wavelength of red light thick—the wavelength of red light is just under twice that of blue, so some red light is reflected. The result is that the transparent film thickens into a metallic white sheen that appears bluer as it gets thicker.

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.

Every integer multiple of a half blue wavelength in thickness, blue light is canceled; every odd multiple of a quarter blue wavelength, blue light is strengthened. Every multiple of a half red wavelength, the red light is cancelled. The result is alternating bands of bluish and reddish light as the film grows thicker—like contour lines on a topographic map.

To further understand how wavelengths move through soap films and to observe how sine waves add up or cancel out, try making our Soap-Film Interference Model.

Robert Hooke first reported observing the transparent film in a letter to the Royal Society. His letter indicates that he thought the film actually did not exist where it was transparent, but that some force held the colored film in place. A simple experiment of poking the invisible film and breaking it proves the existence of the invisible film.

To further experiment with soap films, try drilling a small hole (approximately 5 mm in diameter) in the bottom of the film can. Create a soap film over the open mouth of the can and then blow through the hole. The soap film will bulge out into a dome.

Here’s another cool trick: Try blowing into the can to create a dome and then plug the hole with your finger. If you orient the film can with the opening facing up, colored rings will appear in the soap film.

For more information, read Soap Bubbles: Their Colors and Forces Which Mold Them by C.V. Boys (Dover Publications, 2012).