Physics Demo Number: 132

Approximate Run Time: 10 min

Thin Film Interference Displayed By A Dedicated Reflected Light Apparatus

Demo Description

A dedicated assembly (living in [C-1-4]) allows one to quickly show a real image on the wall of the interference of reflected light from a thin soap film.

The apparatus has been designed and constructed to hold critical values of placement of the components to within an easily reproducible experimental state of affairs.

 

Scientific Principles

  • Interference of reflected light from a soap film.

Equipment

  • Free standing lens-support-assembly

  • Film-bearing wire-loop assembly

  • Jar of soap solution

  • Slide Projector Light Source

 

Equipment Location

  • Kits in [C-1-4]

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2



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Instructions

The first photo shows the usual light source (old slide projector ) on the left.

The free standing lens in a support stand is behind the black rimmed projector lens.

The film-bearing and quarter-dollar-holding wire-loops assembly is seen ( along with a projector-beam baffle) attached to their dedicated free standing support base and column on the right of the photo.

The Science Shop has provided custom instrument making expertise in this assembly.



(Note that a dedicated Quarter Dollar Assembly has been attached to a loop on the film-bearing assembly so that one can easily also show the detailed properties of positive lens image formation, as explained in demo [022]. )

A jar of soap solution sits between the wire loop assembly and the lens assembly so that the demonstrator may raise the jar vertically upwards to immerse the loop in soap.

Subsequent lowering of the jar allows one to arrive at a thin film on the loop.

As the film thins to the point of breaking, one may repeat the raising and lowering of the jar to arrive at a new film.

Note that in this new film generation, one need not disturb the (critical) orientation and spacings among the active elements forming the image of the film on the wall.

The remaining photos show more views of the three elements in their approximate positions during use.

The white foam screen hanging on the coat rack opposite the Faculty Desk ( and seen in the background of these photos ) is a convenient screen upon which to display images in strategical positions in the Lecture Hall.

It sits easily on the chalk tray and tall waste basket (at its bottom corners) in the Lecture Hall.

Writeup created by David A. Burba
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