Am I focusing on the mirror surface, or the reflection?

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Jay Reeve

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Jay Reeve asked the Naked Scientists:
Hi, my question is, when i look into a mirror i have to focus on the objects i am looking at instead of just focusing on the actual mirror itself, if the light is getting reflected off the mirror's surface shouldn't i just have to focus on one certain distance (the mirrors surface)?


What do you think?
« Last Edit: 16/10/2011 03:30:02 by _system »


Offline mitch

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Am I focusing on the mirror surface, or the reflection?
« Reply #1 on: 16/10/2011 03:43:40 »
I would guess it's just a mind-trick.

It's been a LONG time since mirrors and lenses in physics class, but I seem to remember something about how the image you see in the mirror is 1/2 the size of the actual object (because the distance of travel is in a sense doubled?? shrug) Take this with a huge grain of salt as I have no expertise and can hardly remember it:

Assuming the mirror is not convex or concave, when you focus on something in it, you are focusing on what your eyes perceive to be actually farther off in the distance than the mirror itself (twice the distance between you and the mirror). Get a portable mirror, focus on your face in it, then pull the mirror away really quickly, you'll be focused on the objects behind the mirror.

Again, no experience, no expertise. But just some very old education and a little conjecture.


Offline Soul Surfer

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Am I focusing on the mirror surface, or the reflection?
« Reply #2 on: 16/10/2011 10:07:54 »
Jay, You have to focus on the image of the object and not the mirror that is creating the image. For a plane mirror this image is the same distance behind the mirror as you or the object are in front of it.  You can test this out with a little mirror pins and parallax.  Put a long pin or cocktail stick pin in front of a small mirror on a softish surface that you can stick pins into  and look at its image in the mirror and then put a pin at the back of the mirror so that the pin and the image of the pin stay together as you move from side to side.  this shows you where the image really is.
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