Thread: Optics question

Below is an email I sent to my physics advisor. If anyone can provide me a satisfactory answer to my question, I'd appreciate it.

While getting a haircut today, I was examining my haircut without my glasses on and noticed that objects that were far away from the mirror seemed blurry. The mirror was close enough that I could see myself without that image being blurry. If the mirror is essentially acting as the source for the photons that reach my eyes, then why do far away objects seen through a mirror close up appear blurry?

Originally Posted by Mr. White

Below is an email I sent to my physics advisor. If anyone can provide me a satisfactory answer to my question, I'd appreciate it.

While getting a haircut today, I was examining my haircut without my glasses on and noticed that objects that were far away from the mirror seemed blurry. The mirror was close enough that I could see myself without that image being blurry. If the mirror is essentially acting as the source for the photons that reach my eyes, then why do far away objects seen through a mirror close up appear blurry?

It is your eyes, not the mirror...The mirror will not correct your vision...The photons you see reflected in the mirror are going to be the same as if you see them with your own eyes directly.

The mirror is not giving off "photons" as you see them...it is reflecting the photons that are behind you...Your eyes can not adjust to these photons due to whatever problem you have, thus causing them to be blury through the mirror and behind you...

If you are near-sighted (as I am assuming your crisp vision is probably 5-10 feet. looking into the mirror increases the distance between you and the objects behind you by 2-3 feet so everything from 7 feet behind the mirror is probably fuzzy but you are still in focus (your eyes' effective range).

FAW

Funny, I actually seem to know this one.

Assuming the mirror you were looking into was a flat panel, then the light that goes into the mirror is reflected back at its exact same angle, and the exact same distance. The image created of you is the exact same distance from the mirror as you are.

(Object)---(You) -----|-----(Your image)---(Object's image), with the | being the mirror.

So, all objects that appear in the mirror are considered the same distance away from the surface of the mirror as they are in reality. Thus, you would see yourself clearly, but all objects behind you in the mirror would appear blurry because they are more "distant" from your eye.

The light not absorbed by the object travels from the object, to the mirror, then to your eye. For example, our exam rooms at Georgetown are not 20 feet long (the standard for testing vision, ie 20/20) so we have to use a mirror to test at the correct distance.

Your distance vision is blurry most likely because your cornea bends the light too much, when objects are close you are able to use your natural lens (the thing that will eventually become a cataract) to accommodate that refractive error. But when objects are far away, your lens does not have enough power to properly place the light on the retina, this causes your blurry vision.

I believe all of these are perfectly acceptable answers. The take away from all of this is that objects in the mirror are perceived by your mind as being the same distance away from you as they are physically away from you. The wavelength of the light that is reflected from the mirror doesn't change just because you are standing closer to the mirror than the object. There are exceptions of course, such as the mirrors on vehicles as those are specially made to alter the reflection of the objects so that they can show you more of what is directly behind your car.

But the photons hit the mirror, are absorbed by the electrons, which then emit new photons...bah, wish I spent more time in optics class. This is annoying me. I suppose the new photons would have the same energy as the old ones with a high probability. Still, it's annoying.

Thank you for the answers though.

You're thinking about the problem wrong White. It has nothing to do with where the photons originated from, but with where your eye is focused. Your eye will focus on the image in the mirror where the image distance behind the mirror is equal to the object distance in front of the mirror.

http://hyperphysics.phy-astr.gsu.edu...fermat.html#c1