Mirrors and Near-Sightedness

[miguel]

I was hoping someone could shed some "light" on this for me:

If you are near-sighted, far away things become blurry, due to the eye being unable to focus the light entering the eye, right? But things that are closer to the eye focus just fine.

So, if you are near-sighted and you look into a mirror, why do far-away items STILL look blurry? Wouldn't the source of light now be closer (the mirror)?

Am I missing the obvious?

 

[Evadman]

The source of light is still that far distant object. It is just bouncing off the mirror on it's way to your eyes.

 

[CTho9305]

Originally posted by: Evadman
The source of light is still that far distant object. It is just bouncing off the mirror on it's way to your eyes.

put differently, if you traced the path of the rays from the source object to the mirror back to your eyes, you would find that the mirror is effectively not there with regards to how the light enters your eyes.

 

[miguel]

I "see" it now - thanks! So, if a device that regenerates the light were used instead of a mirror, like a tv, then the light source would then be the device and not the object, so the blurry-ness would go away, right?

 

[Evadman]

Originally posted by: miguel
I "see" it now - thanks! So, if a device that regenerates the light were used instead of a mirror, like a tv, then the light source would then be the device and not the object, so the blurry-ness would go away, right?

Well, no not really. That device ( or the TV ) can only focus in on one length. So if it is focussed in on a butterfly, the plane that is 100 feet behind it is still out of focus and will appear that way on the TV. If it regenerates it so that everything is in focus, then yes, that would work.

I did read about a lens that keeps everything in focus about a few years ago, but I have no info on it now.

 

[CTho9305]

i think the way his post was worded, miguel understood it. the whole surface of the TV is at a single distance away from you, so you can focus on it, and its image can be focused on some object. you just have to focus the distance of the front of the TV

 

[Sukhoi]

I think this is similar to why when you look at yourself in the mirror you have to be twice as close to see yourself clearly than when you aren't looking at something in a mirror.

 

[bizmark]

To followup on what Sukhoi said: A diagram always clears things up. Say you're looking at somebody else's nose (your eyes are o's, the nose is 0).

..........0
........./.\
......../...\
......./.....\
....../........\
...../..........\
....o...........o

But now look at your own nose in a mirror:

------------------------
......./\..../\
....../..\../..\
..../.....\/....\
...o......0.....o

The lines represent the path of the light. You see how in the first drawing, the light travelled a certain distance from the nose to your eyes. But in the mirror scenario, the light travels first from your nose to the mirror, then off the mirror to your eyes. So the perceived distance to your nose is actually twice the distance from your eyes to the mirror (assuming your nose was flush with your eyes).

 

[Era]

...............--- ............your nose
................|.....................|
................|.....................|
................|.................... |
...distance |..............--------------- mirror
................|.................... |
................|.................... |
................|.................... |
...............---.... reflection to your eyes


............---................ your nose
.............|....................... |
distance | .......................|
.............|....................... |
............---.......... camera (sharp picture)


............--- ..........monitor (sharp picture)
.............|....................... |
distance | .......................|
.............|....................... |
............--- ...............your eyes


IMHO.

And they call this a WYSIWYG editor. HEHHEH.

 

[Whitecloak]

this formula describes the relation between

distance of object from mirror (u)
distance of image from surface of the mirror (v)
focal length of the mirror (f)

1/u + 1/v = 1/f

for a plane mirror, ie the usual kind, f is infinity. so the magnitudes of u and v will be the same.

hence objects will appear as blurry even when looked at from a mirror

 

[Peter]

And "privilege" is not at all spelt like that 🙂