6D30.20-Thin Film interference


Description: This demonstration shows the interference effects of thin films. If the film survies to a point where it is less then 1/4 of a wavelength of light, no light will be reflected which is characterized by a black part (see picture).


  • 330mm convex Lens
  • 2  +150mm lenses
  • Thin Film apparatus
  • Soap
  • Lowell Pro film light on metal bar
  • Table clamp
  • Jack stand
  • Paper towel
  • Box
  • 3 Portable stands
  • 3 S clamps

Setup Procedure:

  1. This demo takes a while to setup.
  2. Place a jack stand inside the box (to shield students from the bright light).
  3. Place the bubble chamber onto the jack stand.
  4. Wet a paper towel and place it in the bottom of the thin film apparatus. Put the cover on and wait a minute for the chamber to get moist.
  5. Setup the film light to aim at the bubble chamber. Use the table clamp to raise and lower the light.
  6. Put the 2 +150 lenses on portable stands with S clamps and position them in front of each other in the path of the light to focus the light.
  7. Put the +330mm lens on the stand to the side where the bubble reflects.
  8. Setup in the room. Aim it so the reflection of the soap film will go on the screen.
  9. Dip the bubble wand into the soap. Place it back in to the bubble chamber.
  10. By hand, position the lens in front of the reflected light path. Your goal is to try and find a spot where the image looks decent.
  11. When you find this spot, attach the lens to the portable stand.
  12. Make small adjustments until you get the best image possible (see top picture for an example).
  13. The setup should look similar to the above picture.

Demonstration Procedure:

  1. Take careful notice of how the bubble wand is positioned.
  2. Turn the light on.
  3. Remove the bubble wand and dip it in the soap.
  4. Place the bubble wand back into the bubble chamber like it was positioned before. You may have to make slight adjustments.
  5. Some soaps require you to wait a minute.
  6. Note the image on the screen. Explain that the image is upside down (this is why the black occurs at the bottom of the image).
  7. When finished, turn the light off.


  • Different soaps work differently! Some exhibit thin film interference from the start, while others take a minute.
    Even when using the same soap on different days, the patterns may differ. The life of the film also varies, although
    a wet paper towel in the box increases the humidity and increases the life span (sometimes as much as 15minutes).
  • The image produced by the soap film is small, so having a video camera to connect to a projector helps