Skip Ribbon Commands
Skip to main content

6D30.20 - Soap Film Interference

The Web Part cannot find an InfoPath form in the specified location. Either the location does not have an InfoPath form associated with it or it is on a different site collection. Modify the Web Part Properties and select a list or library on the current site.

​6D30.20 - Soap Film Interference

Title6D30.20 - Soap Film Interference

Demonstrate interference patterns caused by a soap film.

Assembly Instructions


Soap Solution:
Quick-pop: Mix the soap and water with very little glycerin. Test the mixture while mixing, when the film pops at ~1 minute it is ideal.
Long-pop: If the instructor wants a film that will stay intact for a while (easier to operate), add extra glycerin. The pop-time will be indefinite.
The students see an image from the reflected light off the soap film surface. To focus that image, we use a large, focusing lens. The light source is a 35mm projector with a circular aperture in it.
Direct the light onto the "sample" surface (ink on a petri dish). Make sure the reflected light is visible on the wall (it should be circular, but out of focus).
Next, put the focusing lens in the path of the reflected light. First do this manually to roughly get the plane where the lens focus's the light. Then, put the lens in a stand and fix it in place so the image is in focus.
Test with a soap film and the lights off.  The film should last approximately 45 seconds to a minute.  If it does not, then the soap solution needs to be modified.
Setup Time15
Operation Time
Preview Time5
Operation Instructions
Operational Steps
1) Turn off room lights
2) Dip the hollow cylinder in the soap and slide onto the fixture, make sure the cylinder ispushed into the fixture all the way.
3) An image of the soap film should appear on the wall.  Because we use a converging lens, the image will be inverted.
As time passes, gravity causes the liquid film to become thinner. When the film is on the order of visible light, you begin to see interference patterns  between the incident and reflected light.  When the film gets less than 1/4 the wavelength of light, a black area is formed due to the destructive interference of the reflections.
Demo on DimeNo
PIRA 200Yes
Export Instructions (if different)


Adding soap (or another surfactant) to water reduces its surface tension, allowing the solution to form films on surfaces or bubbles. We use a hollow tube, and dip it in our soapy solution, creating a film. We then mount that tube such that we can image the surface of the film on a wall. We happen to use a focusing lens which inverts the image (the top becomes the bottom on the wall).
The film is liquid, so the film is thicker at the bottom than it is at the top. The reflected light therefore has different wavelengths for different film thicknesses, and we observe a rainbow on the film.
If the film is sufficiently thin, we will see it turn "black" at the top (which is the bottom on the wall) where it has become very thin just before it pops.

Category6 Optics
Subcategory6D - Interference
Keywordsthin films, soap bubbles
Construction Information
The back end of the cylinder mounting fixture is covered with a piece of black cloth to minimize air current effects while not allowing pressure to build up behind the bubble.
projector slit - small circular
plexiglas cylinder - 2" diameter 4" long, mounted on base
bubble solution
lens - convex 4" diameter, mounted on tripod
petri dish 3.5" for soap solution