General Question

mzehnich's avatar

Why do you need glasses to look in a mirror?

Asked by mzehnich (277 points ) February 15th, 2010

Just got glasses for distance seeing and this has been bugging me. Why do you have the same sight focus when both staring out into the distance, and looking into a mirror that is a couple of feet away? Why are glasses (that normally help you see objects many feet away, out of focus) required to see something that is 2 feet away (i.e. the mirror)?

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7 Answers

MrGV's avatar

Let’s say the object you are looking at is 12 feet behind you, and you hold the mirror 1 foot in front of your face. The light path from the object to your eye is 14 feet.
You would be better off turning around and looking at the object directly, for then the light path would be only 12 feet.The object appears 13 feet behind the mirror, which is 1 foot from your face.

The_Idler's avatar

Distance between you and the mirror is 2x because the light is reflected and bounces back. This is why, if you want to see how you look from a particular distance, you must position yourself at half that distance from the mirror.

mzehnich's avatar

But if poor sight is based on focal distances (i.e. I am near-sighted so my eyes cannot focus well on things far away), how come it doesn’t use the light coming from the mirror as the “source”, thus reducing the distance to your eyes <—> the mirror, instead of your eyes—> mirror—> object the mirror is reflecting?!

Observation-based quantum mechanics?! haha

jerv's avatar

Relativity is a bitch that way :D

stranger_in_a_strange_land's avatar

@mzehnich The light doesn’t originate at the mirror. It is reflecting. That is why your image seen in a mirror appears to be twice as far away. If you are nearsighted, the apparent distance may be far enough to require correction even though the physical distance from your eyes to the mirror is within your uncorrected range.

breedmitch's avatar

You might not need your glasses to see the surface of the mirror, but that’s not what you’re focusing on.

tigress3681's avatar

You are wearing glasses to help you see small things, not things at a distance, per se.

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