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Yesterday I tried on my friend's glasses and noticed that they made everything look blurrier and farther away. If they do this to my vision, how can they make things look clearer and closer for him?

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  • $\begingroup$ For your friend, his glasses make things look clearer AND further away (if he is nearsighted). If his sight isn't too bad then he may not even be aware that things look smaller. Ever notice how a person's eyes look the wrong size through glasses? $\endgroup$
    – James
    Sep 8, 2016 at 18:46

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Glasses are meant to correct defective vision. In some sense your friend's eyes have "somebody else's glasses" on all the time, so the world looks blurry to him. The glasses he wears "undo" the problem of the other glasses and make the world clear again.

The strength of glasses is sometimes expressed in diopters. Perhaps your friend has eyes that have a +2 "error". By wearing -2 glasses, he is back at zero error. But if you, with your good eyes, put on the -2 glasses, you end up with a -2 error, and blurry vision.

This can be explained with detailed physics - but from the question I expect you are looking for this qualitative answer. Please comment if you need more detail.

Some nice diagrams are given on the hyperphysics site - from which I reproduce just one:

enter image description here

To see things in focus (not blurry), the lines that represent the rays of light have to intersect at the retina (the light sensitive surface on the back of the eye). A good lens will do this - a defective lens will either focus in front of the retina, or behind it. An additional lens puts the intersection at the right place. But if you are "correcting" for a mistake that isn't there, you create a problem that wasn't there.

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  • $\begingroup$ No, that's the kind of answer I was looking for. Thanks! $\endgroup$ Sep 8, 2016 at 13:24
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The above answer is good. Thought I would just give a more visual explanation for you. Blurry vision is usually caused by the focal point being either before or after the retina. Glasses move the focal point. If your focal point is correct, putting on your friends glasses moves your focal point to the wrong place, causing blurriness.

Focal point for vision correction

Image from Bing Search: Originally found on http://www.slipups.com but has since been removed from that site.

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    $\begingroup$ If you didn't make that image yourself, you should give proper attribution to its source. $\endgroup$
    – ACuriousMind
    Sep 8, 2016 at 13:44
  • $\begingroup$ Quite right. I have tried to add a proper attribution, not sure if that is right or not. I only added this because for me the image says it all. I just thought it might help clarify the point. Sorry for the omission. $\endgroup$
    – PaulD
    Sep 8, 2016 at 13:58

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