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Name: Jimmy
Status: student
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Question:
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?


Replies:
Hi Jimmy,

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,
Martha Croll


Jimmy,

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
Physics Instructor
Illinois Central College



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