ASK A SCIENTIST
Question: If you are near sighted and looking into a mirror at objects behind you, why are the distant objects seen in the mirror blurry when the mirror is close to you?
A very interesting observation Daniel! I am nearsighted so I tried it myself. Indeed, distant objects in the mirror look quite blurry, although the mirror is just several inches away from my eyes. However, I can see, without my glasses, a far distant mountain printed on a scenic poster if I stand close to it. The mirror and the poster are both on the same wall. Why do objects in the mirror and objects in the poster look different?
We first need to understand what it means to see something clear or blurry. As you may know, your eyeball is an optical device that has a lens and an array of light sensors behind. If a bundle of light rays originating from a single point somewhere outside your eyeball come together to a single point on your retina, you see the point clear. If not, you see it blurry.
An important thing to note is that the entrance into your eyeball (called "pupil") is a round hole with a certain size. This causes some variation in the direction of light rays that go through it. Imagine light rays coming to your eye from a single point in the environment. It is not so hard to understand that the rays that pass near the upper end of the pupil go slightly upward, while those that pass near the lower end go slightly downward, etc. This variation is large if the point is close to your eyes, or small if it is far away. The lens of your eye tries to refract the rays appropriately so they are all projected to a single point on your retina.
Now we are ready to address the original question. The difference between objects in the mirror and objects in the poster is how much variation exists in the direction of light rays going through your pupil. The objects you see in the poster are made of light rays that are reflected from pigments on the surface of the poster, which is very close to your eye, so they have large variation of direction when they go through your pupil. Our nearsighted eyes are good at correcting such large variation. In contrast, the objects you see in the mirror are made of rays that come over a much longer distance from behind you, so the variation of their direction is smaller. They wouldn't have to be refracted much, but still, our nearsighted eyes can't help but refract them too much. This is why the objects in the mirror look blurry to us.
Finally, I would like to mention a simple, fun experiment. Have a light-proof heavy paper and make a tiny pinhole on it using a pushpin. Put your glasses off and look at something through the pinhole using one eye, with the other eye closed (this works best under a lot of light, like outside on a sunny day). You will be surprised to find how clearly you can see things, both far and near, no matter if you are nearsighted or farsighted! Why does this happen? Think about the reason using what you have learned from this column. Enjoy!
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