HOW COME?: Mirrors only reverse one way

When we see objects in a mirror, how come they're reversed left-to-right (but not up-to-down)asks reader Ray West.Ever try to read the slogan on your T-shirt in a mirror? That's why emergency vehicles print their names backward: Your rearview mirror handily makes them readable again, helping you to get out of the way.

But a mirror only seems to reverse left and right. One clue is that a mirror doesn't flip images top-to-bottom. If it did, shaving, brushing teeth and applying makeup would get awfully tedious. (Not to mention dizzying, since a full-length mirror would picture us hanging neatly upside down.)

What an ordinary flat mirror does is reflect the light bouncing from what is presented to it -- in straight lines. So the left side of your face appears where it should, on the left side of the mirror, with the right side on the right. And, thankfully, the top of your head is reflected back at you from the top (not the bottom) of the mirror.

The fact that a mirror reflects back what is in front of it makes reading in a mirror a frustrating exercise. Turn the cover of a book around to face a mirror and it will reflect the title back, letter for letter, left to right. Presto: We're trying to read backward.

To read "backward" writing in a mirror, you'll need a second mirror, held up to the first. You can also use a second mirror to see your own true image. Simply set two mirrors at a right angle, like two walls that meet in the corner of a room. Or hold a hand mirror at a right angle to a bathroom mirror. Since we're so accustomed to seeing our own face in mirrors, it can be jarring to see the subtly different face that others actually see when they look at us. Despite photos and videos, it's startling to see, close up, and in real time, what we look like to others -- hair parted on the opposite side, crooked smile and all.

Still, it seems like a paradox: Wave at yourself in the mirror with your right hand, and your reflection will obligingly wave back at you -- with what seems to be your left. But according to physicists, it's our interpretation that's off. A mirror actually reverses "in and out," which is why a pencil pointed at the mirror points back at you. So to truly put yourself in your mirror image's place, you would have to turn your back to the mirror.