Demos: 7A-03 Left-to-Right Reversal

It is commonly misstated, even in some texts, that a plane mirror reverses an image right-to-left. This misconception results from our interpretation of what we see when we look in a mirror, because we are bilaterally symmetrical (for the most part). This demonstration helps clarify the concept that a mirror reverses an image front-to-back, not left-to-right.

Directions: Place the large plane mirror on the lecture table. Face the mirror with your back to the audience, and move your hand to the right, allowing the students to see the image of your hand as it does so. Then move your hand to the left, allowing the image to be seen. Finally move your hand toward the mirror, again allowing the image to be seen.

Hold the transparency, on which the word CAR is printed, toward the students. The transparency should be backed with a sheet of white paper. Then turn it around and face it toward the mirror. The word CAR is apparently reversed left-to-right. Without moving the transparency, take away the white paper so that the word CAR can be seen on the transparency. Now it is evident that no reversal occurred.

Suggestions for Presentation: This concept is a difficult one to get across because students have for so long believed that a mirror reverses right-to-left. Ask the students if they were to lie down in front of a mirror, would their head and feet be reversed too?

In the first demonstration, show that when you move your hand right or left, the mirror image moves in the same direction. But when you move your hand toward the mirror, the image moves in the opposite direction! Notice also that when your image appears to have been reversed left-to-right, the pocket on your shirt (or the mole on your face, etc) does not reverse sides. It is only because humans are bilaterally symmetrical that we think there is another person facing us and that person is turned around.

The second demonstration at first seems to prove that the lettering is reversed right-to-left. However, after letting the students read the sign, you then turn around face the mirror. Now you have rotated the image 180 degrees! By pulling the backing paper away, the students can now see that both the transparency and the image are reversed.

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