Teaching Physics with the Physics Suite

Edward F. Redish

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TV versus mirror

Suppose that you are setting up a laboratory to study the decision making behavior of rats. You will have them running a maze and will put wires in their brains while they run it in order to measure what they are thinking as they make choices about where to go.

Suppose also that you want to outside the room so as not to distract them while they are doing their tasks in case you need to intervene for some reason. You can think immediately think of two straightforward approaches. (1) Put up a mirror in the room so you can watch them while you are in the next room. (2) Put a webcam in the room so you can watch them from anywhere.

There are various advantages and disadvantages to these choices, but you get distracted from making a decision by the following interesting question. In both cases, an image of the same volume of space containing the same objects is displayed on a two-dimensional surface -- in one case the surface of the computer screen, in the other the surface of the mirror. Yet there is an interesting difference. If you move away from looking at the center of the computer screen and go off to the side, the objects on the surface of the computer appear distorted -- squashed along the horizontal direction. (This is what it means to be a 2D image.) On the other hand, if you walk off to the side of a mirror, nothing is distorted. Everything looks normally 3D -- you just see somewhat different parts of the room.

Explain why this difference occurs in terms of the physics of light and mirrors as you have learned them in class.

This problem has an interesting relation to what you have to do to create 3D TV. A cool video on the topic is on You Tube. (Thanks to Nick Cummings for this link.)

Page last modified February 16, 2011: OP43