Built on Facts

Yesterday I taught the basics of geometric optics to my Physics 202 students. We did plane mirrors, spherical mirrors, and thin lenses. All told it’s a fairly straightforward chapter that’s all theme and variations on one equation. The sign rules for object and image distances have to be remembered correctly, but that’s not so difficult.

Now while students aren’t necessarily going to come into class with previously built intuition for Ampere’s law and all the rest of the fun equations of the electromagnetic fields, they do have many hundreds of hours of experience looking in mirrors. All of them know how mirrors work. Or at least I’m sure they all think so!

But even simple things can have subtle complications. I’m thinking about giving this as a quiz question:

Why do mirrors reflect right and left, but not up and down?

Give it a try!

Comments

#1 Douglas
July 16, 2009

ubevmbagny fgrerb ivfvba[rot13]
#2 Max Fagin
July 16, 2009

“Why do mirrors reflect right and left, but not up and down?”

Umm, they don’t, do they? When I’m looking in a mirror, my left hand appears on the left side of the mirror, my right hand on the right side, and my head an feet at the top and bottom respectively.
#3 Bexley
July 16, 2009

They dont – they reflect forward and back.

Stand in front of a mirror and move your hand up and the reflections hand moves up too; move it right and the reflections hand moves to your right; however if you move your hand forward (away from you) and the reflections hand moves towards you.
#4 Bexley
July 16, 2009

Previous answer assumed that the mirror is set out in front of you like in the picture you have.

I guess why people are confused is because of the way we think about our left and right sides. We can only think about left and right once we have defined front/back and up/down. Since a mirror in front of us reflects front/back we have the illusion of right and left being reflected. When we wave our right hand the reflection waves the hand to our right but we think of it as the reflections left hand.
#5 NJ
July 16, 2009

Because then it wouldn’t be a mirror operation, it would be an inversion center!
#6 Brian H
July 16, 2009

@Bexley

Exactly, and text appears to have been reversed because you turn it around to point it at the mirror! If you hold a transparency with the text correct to you up to a mirror, the text will also be correct in the mirror.
#7 Gray Gaffer
July 16, 2009

trick question. When I look in the mirror I am seeing (mostly) what is behind me. To see it directly I must rotate 180 degrees about a vertical axis. Hence the left/right inversion.

compare and contrast with the directionality of a focussed image, as at the prime focus of a telescope.
#8 Robert
July 16, 2009

I think that is caused by the position of our eyes. Would they be ordered vertically we would see an upside down reflection
#9 Paul Murray
July 17, 2009

@3 beat me to it!

But the reason for the error is that when working out what’s different, we mentally place ourselves where the reflection is – standing upright, but facing us.
#10 Max
July 17, 2009

When I was 8 or 9 years old the notion got in my head while I was sitting in the attic staring at a full length mirror. Took me at least 30 minutes if I recall correctly. Then I moved onto concave surfaces like spoons. Thanks for bringing back the memory.
#11 Gerry
July 17, 2009

True story: My college physics instructor responded to a question about mirror reflection by saying: “Think of leaning over and looking between your legs. What do you see?” He had a great deal of trouble getting the class back in order.
#12 Uncle Al
July 17, 2009

http://www.mazepath.com/uncleal/invert.gif

Two plane mirrors intersect at right angles. The center reflection is double revesed – which is to say, unchanged. Buy a good suit and enjoy the shop’s mirror.
#13 Anonymous
July 17, 2009

The reverse neither. They reflect back and front.
#14 bad Jim
July 17, 2009

We think a mirror reverses right and left because we typically rotate an object around a vertical axis in order to view it. If instead you rotate a book around a horizontal axis you will find the text reversed vertically instead.
#15 Neil B ♪
July 19, 2009

Yes, commenters are “right” and I’ve made the same points. However, some people say that the image reverses the handness per the standards of the “reversed world” – that “right hand” you wave is still “on your right” in the mirror, but it’s a “left hand” relative to the direction the “mirror image you” is facing. Wow, even more philobabble than the fallacious attempt to use “decoherence” to solve the collapse problem.
#16 William
July 19, 2009

Feynman’s explanation
#17 ewkk
May 6, 2011

I honistly hate it so go get a real job.