Day 67: Inverse Square Law


This is in response to a question from physics teacher @writehandrule on Twitter:

I have found some success using the gravitation simulation from The Physics Classroom website and the accompanying activity that helps students “derive” the inverse square relationship.

Upon further pondering, I wondered if introducing the invervse square law for light first (a more concrete inverse-square law, in my opinion) would be more effective. So I rigged together this little demo involving a grid of dots drawn on an overhead projector.

In the video, fellow physics teacher Mr. Longhurst, stands 1 meter in front of the projector. You can count 64 “light beams” (8 dots x 8 dots) on the mini-whiteboard he’s holding. Then he moves to stand 2 meters from the projector; now there are 16 light beams on the WB (4×4). Finally he moves 4 meters away, and there are 4 light beams on the board (2×2).

If you’d rather an activity for small groups, you might be able to pull it off very cheaply with just diffraction grating “rainbow” glasses and a red laser pointer. (The rainbow glasses produce a grid of light beams rather than just a line of beams like a regular diffraction gratings.)

About Frank Noschese

HS Physics Teacher constantly questioning my teaching.

3 responses to “Day 67: Inverse Square Law”

  1. Trianglemancsd says :

    Very cool. My inner eighth-grader was wondering, "Why are the ‘light beams’ dark dots?" and this led me to think about punching holes in a piece of paper so that it’s really light beams we’re counting instead of "absence of light beams". But this is a minor tweak to a delightful demo.

  2. Anonymous says :

    Because that’s what I could whip together in 2 minutes after school to get this posted ;)But, you’re right. Ideally, I’d hole punch paper (or use a piece of pegboard) to create actual light beams rather than dark dots.

  3. Anonymous says :

    And before doing the demo in the video, I highly recommend do the following demo FIRST: Use the rainbow glasses w/ laser AND using chalk dust/water/party fog to illuminate the laser beams so kids can SEE the beams spread out in all directions.

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