Concepts

Thin film interference, reflection phase change

Overview

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.

Details

Equipment

• [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] Projector screen

Classroom Assembly

1. Unfurl the projector screen.
   
2. Mix around the soapy solution a bit.
   
3. Pour soapy solution into beaker.
4. Mount copper ring above soapy solution, with beaker cover in place.
5. Mount lens on lab stand.
6. Plug in projector, turn it on, and shine light at the copper ring.
7. Make soap film by immersing copper ring in soapy solution.
8. Align lens so that reflection of the soap film is projected onto a visible area. Avoid beaker glare if possible.
9. 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.

Script

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, turn the projector to fan mode.

Additional Resources