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).
Equipment:
- 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:
- This demo takes a while to setup.
- Place a jack stand inside the box (to shield students from the bright light).
- Place the bubble chamber onto the jack stand.
- 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.
- Setup the film light to aim at the bubble chamber. Use the table clamp to raise and lower the light.
- 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.
- Put the +330mm lens on the stand to the side where the bubble reflects.
- Setup in the room. Aim it so the reflection of the soap film will go on the screen.
- Dip the bubble wand into the soap. Place it back in to the bubble chamber.
- 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.
- When you find this spot, attach the lens to the portable stand.
- Make small adjustments until you get the best image possible (see top picture for an example).
- The setup should look similar to the above picture.
Demonstration Procedure:
- Take careful notice of how the bubble wand is positioned.
- Turn the light on.
- Remove the bubble wand and dip it in the soap.
- Place the bubble wand back into the bubble chamber like it was positioned before. You may have to make slight adjustments.
- Some soaps require you to wait a minute.
- 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).
- When finished, turn the light off.
Note:
- 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
28-N4
