How much can you see of yourself in a mirror?

by Stephanie Chasteen on November 13, 2011

This is a nice take on the standard physics “how much of yourself can you see in a mirror” activity.  Another tip of the hat to Karen Hunter at the Oregon AAPT meeting for this one.

So, to refresh your memory, if a mirror is 1 foot tall, you can see two feet worth of yourself in that mirror.  So, to see your full body, you would need to use a mirror that is half your height.  Here’s a ray diagram demonstrating that, shamelessly stolen from Cliff

So, Karen had a very nice way to teach this, using a body experience, a prediction, graphical representation, and then checking via experience.

  1. Students stand in front of a mirror.  Have them line up the mirror so that the top of the mirror aligns with the top of their head.
  2. Have them place a piece of tape on their body at the lowest point they can see in the mirror.
  3. Sit back down and put away the mirror.
  4. Now ask them, how could you see more of yourself in the mirror? (They’ll typically say that you’d have to back up)
  5. Give them graph paper with the mirror drawn on the paper, and two stick figures at two different distances
  6. Have them draw ray diagrams to show how much each person can see of themselves in the mirror. They should find that it will be the same.
  7. Now have them check their prediction.  They’ve still got the piece of tape on them from before, what happens as they back up?

I liked this, as I say, because it keeps the element of suspense, asks students to predict a real-world phenomenon that they can actually check, and they use physics and sketching to predict it.  It’s a really nice little lesson.  I can imagine using this in a large lecture class too — just do it with two students as demo students, and then have each student do the graphing activity.


JJ December 10, 2012 at 11:28 pm

When you back-up why can you see more of yourself in the mirror?

Stephanie Chasteen December 11, 2012 at 7:51 pm

JJ, have you tried it? You can’t see anymore of yourself as you back up. Try it and see. As I stand farther from a mirror I see more of my surroundings, and the image of myself gets smaller, but I can’t see any more of myself. That’s the point of the post.

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