Do Mirrors Really Flip Images?

Adam Beehler

In each newsletter, Adam Beehler, Lecture Demonstration Specialist, explains one of his
demonstrations. Adam recently authored two articles, published in The Physics Teacher:
•“Demonstrating the photoelectric effect using household items” (Vol. 48, 348. 2010)
•“Demonstrating spectral band absorption with a neodymium light bulb” (Vol. 48, 206. 2010)


If you were to go up to some random person and ask her if mirrors flip images, what do you think she would say? Whether or not they actually understand what is going on with the physical light rays, most folks will agree that their image looks reversed. Well, it is a misconception that plane (flat) mirrors flip images. Indeed, when you view yourself in the mirror, the image you see is reversed left-to-right; however, it is not the mirror that does the flipping.

Let’s look at an example. In the light ray box picture there are five rays exiting the box to the right. They then reflect off of a plane mirror and continue on their way down to the left. If you were to look at the light source, you would see the letter “A” on your right. Please note that your face would be to the right of the box and you would be facing left in order to see the rays coming out of it. Now, in order to see the light rays reflected off of the mirror, you must move to the lower left and face right, looking at the mirror. What side is the letter “A” on now? Ahh, the image has reversed! The letter “A” is now on your left. So what did the actual flipping? Trace light ray “A” coming out of the box and off of the mirror. See how it stays on the same side (the top in our picture’s view). The mirror did not do the flipping. It was us. We are the ones who turned around.

Mirrors do not flip images left-to-right nor top-to-bottom, but they do flip images front-to-back. This can be seen (pun intended) by facing a mirror. You face in one direction yet the image of yourself is facing back at you in the opposite direction. If the image was not flipped front-to-back, then you would see the back of your head. The following images are some intriguing ways to prompt discussion of reflections from plane mirrors. The jack-o-lantern was my creation one Halloween.

You can also view this demo, and a complete materials list, here.