[Virginia Tech Department of Physics]

 Lecture Demonstrations
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L40: Thin Film Interference

 

Purpose: To demonstrate interference in the light reflected from the two surfaces of a thin film.
Description:

 

Place a soap film over the end of the Lucite pipe. Shine the light beam on the film and align the lens to focus an image of the film on a nearby projection screen. After allowing a few minutes for the film to stabilize, you will have an inverted image something like the one pictured.

  Notice the small black region of destructive interference at the top of the film (bottom of the image) where the film is very thin. (This region gets larger with passing time. The larger dark region at the bottom of the film results because the film is too thick to give distinguishable fringes.) With a clear image, so that you can count fringes, you might do a quantitative calculation of the film thickness at some point. Otherwise, just ooh and aah!
Equipment List: Storage Location: CHEMP 130A

 

 Slide projector or other source of light beam A2.3
  Soap solution A2.3
  Soap roller and tray H1
  Lucite pipe to hold film, lens. A2.3

References:

Manual: None   Setup Notes: L40 PIRA #:
Manufacturer(s): Other school's Demonstration web pages

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