Why do mirrors reverse right left and not up down?
A large mirror, as big as a person
thin cardboard, bigger than a hand
a red dot, stick on
a green dot, stick on
Put your hand on the cardboard with the fingers
Trace an outline of your hand with the pencil.
Cut out the outline of your hand.
Preliminary activity Right and Left
To Do and Notice
Show the hand to a group of people and ask them,
"is it a left hand or a right hand?"
I usually say "I have a very simple question for you," as a warning to them that I am about to ask a trick question.
You may get different answers from different people.
From the cutout it is impossible to tell if this
is a right hand or a left hand.
While the cutout has fingers and a thumb, as any self respecting hand would, it does not have a palm or hairy knuckles.
Ask people what they feel when they think right or left, some people feel that hand, other people feel that foot, but in general they think about a side of their bodies. This may lead to confusion later.
What's Going On?
If the fingers point up and the thumb is to the side you cannot determine whether it is a right hand or a left until you know which side the palm is on. It takes three different directions to determine a hand, the direction of the fingers, the direction of the thumb and the direction of the palm.
Right and left are complicated concepts.
Take both hands and point the fingers toward the
Point both thumbs toward the same wall.
One palm will point toward you and one will point away.
To Do and Notice
Stand in front of a mirror looking into the mirror.
Put the red dot on the top of the mirror and the green dot on the bottom.
Put a hand near the red dot and wiggle it. Notice that the image person in the mirror also wiggles their hand near the red dot.
Put the other hand near the green dot and wiggle it. Notice that the image person in the mirror also wiggles their hand near the green dot.
Notice that the mirror does not reverse the green dot and red dot hands of the image person when the dots are put on top or on the bottom of the mirror.
Put the red dot on the side of the mirror near your right hand and the green dot on the side near your left hand.
Wiggle your right hand near the red dot. Notice that the image person wiggles its hand near the red dot.
Wiggle your left hand near the green dot. Notice that the image person wiggles its hand near the green dot.
Notice that the mirror does not reverse the red dot and green dot hands of the image person when the red dot is placed on the side of the mirror and the green dot is placed on the other side.
(This may surprise you since people often say that mirrors reverse right and left but don't reverse up and down. But the observations you have made are correct so what is going on here?)
Now place the green dot at the bottom center of the mirror and the red dot 1 meter in front of the center of the mirror.
Point toward the mirror, you will be pointing from the red dot toward the green dot.
What is the image person in the mirror doing. They are pointing the other way, from the green dot to the red dot. (Ignore the image of the red dot in the mirror.)
So the mirror does reverse in and out.
Point a flashlight at the mirror, the reflected beam comes out of the mirror.
You point away from yourself the image points toward you.
If you point into the mirror toward the east, the image points out of the mirror toward the west.
What's Going On?
The mirror reverses in and out, it does not reverse up and down, it does not reverse side to side.
This is enough however for the mirror to create an image of a right hand that is a left hand.
Hold up your right hand with the palm toward the mirror, away from you. With the fingers pointing toward the ceiling. put a green dot on the side of the mirror nearest your thumb.
Look at the image hand. The image fingers point toward the ceiling, the image thumb is nearest the green dot, yet the image palm points out of the mirror toward you. This reversal of one direction makes the right hand turn into a left hand. This reversal is why mirrors are said to turn right into left.
Two Mirror Activity
Find two large mirrors at right angles.
Look at your image in the mirror.
Does this mirror turn a right hand into a left hand?
Hold up your right hand with the fingers up and the palm toward the mirror.
Mark the side of the mirrors nearest your thumb with a green dot and the other side with a red dot.
Look at the image hand. The fingers on the image hand point up.
The thumb on the image hand, however, is near the red dot side, it is on the opposite side from the thumb on your real hand.
The palm of the image hand points out toward you while the palm of your real hand points in to the mirror away from you.
The reversal of two directions keeps the right hand a right hand.
Spherical mirror activity
Find a large spherical mirror.
Reach into the mirror and shake hands with yourself!
Notice that your fingers point in to the mirror, the image fingers point out.
Your palm faces the green dot side (put a green dot on the palm side) the image palm faces the red dot side.
Your thumb points up, the image thumb points down.
What's Going On?
All three directions have been reversed. This turns the right hand into a left hand. Carefully look at the hand you are shaking, notice anything strange? You are shaking the wrong hand, the thumb is on the wrong side.
Scientific Explorations with Paul Doherty
27 Aug 99