Location of Plane Mirror Images
If I stand close to a mirror with my glasses off, I can
see the mirror itself as an object very clearly. However the things
reflected in the mirror are still blurry, as if I were to be looking
directly at them without glasses. Why is this so?
In a flat bathroom mirror, also called a plane mirror, the image you
see is actually a virtual image behind the mirror surface. In other
words, the image is the same distance away from the mirror surface
as the object. If you are standing close to the mirror without your
glasses, and are nearsighted, you will see yourself and the other
nearby objects in focus. If you need your glasses to see things far
away, like I do, anything that would normally be blurry in your
field of vision will still remain blurry. If you put on your
glasses when looking in the mirror, the far objects should appear in focus.
Remember, on a flat mirror, the images are as far away from the
mirror surface as the objects are from the mirror. There are really
no images on the mirror surface, unless you touch the mirror. It is
kind of a weird thing to think about. But put on your glasses and
look in the mirror at the far objects and see if they do not clear
up. It is just like looking at the far away objects without glasses
and then putting your glasses on and looking at them again. You can
also try this with a hand held mirror outside, glasses on glasses off.
Thanks for your question,
When you look at an object in a plane mirror (a flat mirror), some of
the light coming from the object bounces off the mirror's surface and
then reaches your eye. This light is what registers as you seeing the
object. These light rays that reach your eyes from the reflection are
the same as the rays you would see from the original object if you
removed the mirror and placed the object not where the mirror was but
quite a way behind where the mirror was. If the original object was
five feet in front of the mirror, the reflected rays are the same as you
would get from an object five feet BEHIND the mirror.
Your eyes see the light from the object, not the object itself. Your
eyes respond to a reflection the same as they would respond to a real
object that produced the same pattern of light. This is what we mean by
the location of the image. Where would a real object have to be to give
me a light pattern that matches the pattern I see from this reflection?
Dr. Ken Mellendorf
Illinois Central College
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Update: June 2012