[Physics] Reflections in Rearview Mirror

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In the rear view mirror of your vehicle, the driver in the car behind is on the driver's side and the passenger is on the passenger's side. When they raise their left arms they are on the same side as your left arm. Why are the only things reversed the numbers and letters on license plate and the name of the car?

Best Answer

Contrary to piles and piles of common and well-intentioned explanations, a mirror doesn't reverse left and right. A mirror reverses towards and away.

If you are in your car facing to the north, and the driver behind you sticks out his left arm on the west side of his car, what you see in the mirror is a driver facing to the south with an arm stuck out on the west side.

We ordinarily describe this as a reversal of left and right because we use a cross product to define our relative directions. There are two choices for the direction of a cross product, distinguished by whether you use a "right-hand rule" or a "left-hand rule." But reflection in a mirror turns a right hand into a left hand, so images in mirrors disagree with us about the mapping of (up, forward, left) onto (up, north, east-west).

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