Why is it, that short sighted people cannot see an image clearly in a mirror of a reflection far away?

Question

Say you have moderately bad eyesight whilst you are driving, and you look in your rear view mirror which is only a foot away from you. Why is it that you cant read a number plate that is physically meters away - when the reflection is but feet infront?

Answer 1

the light traveling from the object you are looking at in a mirror still has to travel the distance you are standing from the mirror plus how far it is from the mirror, so the image still gets focused on the wrong part of the eye and is out of focus as if it was farther away.

mirror 1 foot away

license place 20 feet away

distance for light to travel 21 feet

so it will be focused the same as a license plate 21 feet strait ahead of you.

Answer 2

because the light still has to travel from the object to your rear view before you can see the object in it

same distance if not slightly further

Answer 3

Imagine looking at a tree 50m behind a window, say 1m away.

Light passes through the window. You aren't seeing the image on the window, you are seeing the tree behind the window. Your eyes still have to focus 50m away to see a tree.

A mirror is no different, except that it changes the direction of light. You aren't seeing the image of the tree 1m away in the window, you are seeing it 50m away. Your eyes still have to focus on a point 50m behind the mirror in order to see a tree in focus.

Now, if you replaced the mirror (or window) with a TV camera and screen, then the light is emitted from the same plane (the TV screen) and would be in focus at 1m. This is also why you loss the 3D nature of what you are looking at. Every point on the TV screen is exactly 1m away and you can clearly see anything by only focusing your eyes to 1m.

Answer 4

The reflection in the mirror is not the same as a picture of an in-focus plate that is just inches from your face. The mirror simply changes the angle of incoming light to reflect it back to you. It doesn't change the total distance the light travels from 20m to 20cm, it just bends it for the last 20cm of the journey.

The focusing of your eyes is entirely dependent on the distance from the object reflecting (or emitting) light, assuming there are no intervening lenses. This should be clear if you attempt to focus on a speck of dirt that is actually on the surface of the mirror. Only then are your eyes actually focusing for an object that is 20cm away.

Hope that is helpful.

Answer 5

Because, even though the mirror is 1-2 feet away from your face, the image it is reflecting is still far away.  So when you think about it,  a near sighted person looking at his rear through the rear-view mirror, would be like just looking at it with out the mirror.  (Sorry if it was a little redundant.)