Blurry Mirror on the Wall

Your Smartness:

I'm nearsighted, and when I look in the mirror without my glasses, I can see myself okay but things across the room look blurry. But if the mirror is close enough for me to see my own reflection clearly, then why can't I see the reflection of things across the room? I'm seeing both reflections from the same distance. I'm sure you can clear things up.

-- Four Eyes, El Cajon

Yeah, I can. But you're not going to believe me anyway. Nobody ever seems to buy this story, so here goes nothing.

We see objects by focusing the light that bounces off them and travels to our eyes. To say you're nearsighted means your eyes can't focus light that travels farther than a certain short distance.

When you look at a reflection in a mirror, and the object is eight feet from the mirror and you're, say, two feet away, you're not seeing light coming from just two feet away. You're seeing light that has traveled from the object to the mirror, has reflected from the surface of the mirror, and then traveled another two feet to your eyes. If your eyes can't focus light that's traveled ten feet, they won't be able to focus light that has traveled eight feet to the mirror and two feet more to your eyes. In terms of distance, the image in your mirror is an exact duplicate of the reality it is reflecting.

Ever wonder how an eye doctor can test your distance vision in his tiny examining room? He puts the eye chart behind you, then projects the chart onto a mirror across the room. That mirror reflects the chart onto a mirror behind you, and so on. The light zigzags back and forth among a series of mirrors, and the final reflection is directed toward your eyes. By the time you see it, the light from the chart has traveled several times the length of the room. Otherwise, he would have to give you an eye test in a bowling alley.

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