6D30.20 Soap Film Interference


Thin film interference, reflection phase change


Light reflects off a soap film to create an interference pattern. When the soap film is first made, the film is relatively thick, creating a colourful pattern, largely from constructive interference. Over time, the film thins out, making the reflection black due to destructive interference.



  • [1] Soapy solution
  • [1] Beaker
  • [1] Beaker cover
  • [1] Rotatable clamp
  • [1] Clippy holder
  • [1] Copper ring
  • [2] Lab stand
  • [1] Projection lens
  • [1] Lens holder with chain
  • [1] 90-degree clamp
  • [1] Slide projector containing slide with aperture
  • [1] Extension cord
  • [1] [AV] Projector screen

Classroom Assembly

  1. Pour soapy solution into beaker.
  2. Mount copper ring above soapy solution, with beaker cover in place.
  3. Mount lens on lab stand.
  4. Plug in projector, turn it on, and shine light at the copper ring.
  5. Make soap film by immersing copper ring in soapy solution.
  6. Align lens so that reflection of the soap film is projected onto a visible area. Avoid beaker glare if possible.
  7. Turn the projector to fan mode.

Important Notes

  • When done, the projector should be put in fan mode to allow it to cool off for a few minutes.


  1. Turn on the slide projector.
  2. Briefly immerse the ring in soap solution.
  3. Discuss the interference pattern as it appears.
  4. After a while, the thinnest part of the film will reflect no light, turning black.
  5. When done, turrn the projector to fan mode.


Additional Resources


  • PIRA 6D30.20


  • Don't attempt this at home!

Last revised

  • 2019


  • Reflection off the front surface of the film creates a π phase shift.
  • The colour pattern appears to be primarily due to constructive interference of reflected rays, with a repeating pattern of purple/blue to green to orange/red.
  • The interference pattern is inverted by the lens.
  • AV Services can lend a screen to allow viewing of the soap film reflection at chest-height. The interference pattern does not project well to greater heights.
  • Make sure to leave a little extra time for fumbling with the screen.

Related AV

Related demos

  • ?


If you have any questions about the demos or notes you would like to add to this page, contact Ricky Chu at ricky_chu AT sfu DOT ca.