Reflection on a Pond has a One-to-One Correspondence; It Doesn't Flip Horizontally

So we were talking about plane mirrors the other day. What makes mirrors so special that they apparently only only reflect left and right and not up and down? After some discussion, we jokingly decided that  it was because of gravity, because one way or another, we can usually blame gravity for just about anything.

We’re kidding.

Some people say it’s the eyes or brain that interprets a reflection from a mirror the “proper” way we see it. This assertion cannot be true. While it is true our eyes gathers light and inverts them in our eyes (like with a concave lens, for example), our brain interprets it inverted as well, which makes the image “upright” in our minds. The eye is simply a light sensor; they way it gathers light is not a function of what the light source is (it doesn’t change the way it takes in light depending on what you’re looking at).

Actually, plane mirrors are simpler than most people probably make it out to be. Let’s try this. On a piece of paper, write the word “TEST”. Now show it to the mirror to read its reflection. As we expect, the letters are reflected horizontally. Now, write the word “TEST” on a semi-translucent piece of paper. Hold up this paper up to the mirror, but this time have the word facing you. Now, we see that you can also read “TEST” the proper way in the mirror as well; it’s normal!

From this test, we can see that mirrors reflect left to the left and right to the right. Now, how does this work? Consider meeting someone face-to-face and shaking hands. In a right-handed society, you’ll extend your right hand and the other person will extend his right hand. To you though, he appears to be extending from his “left” side. The same idea applies to plane mirrors. The only reason a mirror appears to reflect horizontally is because you actually turned to look at it (as if walking up to meet your virtual self). In other words, the reason why objects (or letters) look reversed in a mirror is because you are presenting them to the mirror already in reverse.

Try it! It’s cool.