Physics Help Forum problem with plane mirror

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 Oct 27th 2009, 02:04 PM #1 Junior Member   Join Date: Sep 2009 Posts: 22 problem with plane mirror i encounter a junior physics question today, but I am not quite sure if I think correctly. The question is something like..... There is a mirror hung on some height of the wall, a person stands in front of the mirror and look at the mirror, where he can see his the image of his neck, and also the image of his wrist... something like that. As he moves away from the wall, while the position of the mirror stays unchanged, how different the image will he see in the mirror? i.e. will he see MORE part of the image of his body or will he see LESS part of the image of his body? Forgive my bad english, try to understand please. Thanks
 Oct 27th 2009, 10:19 PM #2 Physics Team   Join Date: Feb 2009 Posts: 1,425 I think that if we move away from a mirror, we see more of our body. To actually show it you will have to make a ray diagram for both the cases. Google "reflection plane mirror ray diagram" and you will find plenty of stuff.
Oct 28th 2009, 09:24 AM   #3
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 Originally Posted by physicsquest I think that if we move away from a mirror, we see more of our body. To actually show it you will have to make a ray diagram for both the cases. Google "reflection plane mirror ray diagram" and you will find plenty of stuff.
why
but i think we will see the same amount of object

why we can see the image of hand in the mirror, is because there is light from our "object" hand goes to the mirror and then reflect to our eyes
as we move away from the mirror, the distance between our eye and our hand remain unchanged, so there is still light from our "object" hand goes to
the mirror which can be reflected to our eye. Only difference is the angle of incidence and reflection from the mirror

Am I right?

Last edited by kenny1999; Oct 28th 2009 at 09:41 AM.

 Oct 28th 2009, 09:52 PM #4 Physics Team   Join Date: Feb 2009 Posts: 1,425 When we consider a mirror, it also has a finite size. Suppose at a particular distance from the mirror, you can just see your knee, but nothing lower down. This means as you said that the ray of light from the knee is reflected from the mirror (its lower end in fact) and falls on your eye. If you now move closer to the mirror, the ray from the knee will have a larger angle of incidence and reflection and miss youe eye. However, the portion just above your knee might have the correct angle to be seen. Hence we see smaller portions as we move towards the mirror. Please google and check. It is difficult to visualize without ray diagrams.
Oct 29th 2009, 10:39 AM   #5
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Posts: 22
 Originally Posted by physicsquest When we consider a mirror, it also has a finite size. Suppose at a particular distance from the mirror, you can just see your knee, but nothing lower down. This means as you said that the ray of light from the knee is reflected from the mirror (its lower end in fact) and falls on your eye. If you now move closer to the mirror, the ray from the knee will have a larger angle of incidence and reflection and miss youe eye. However, the portion just above your knee might have the correct angle to be seen. Hence we see smaller portions as we move towards the mirror. Please google and check. It is difficult to visualize without ray diagrams.

I think you wrong

as the observer move closer to the mirror, his knee will , of course , have a large angle of incidence

but don't forget he has moved closer , so the distance between himself and the plane mirror is reduced.

see my diagram, which i draw using snagIt.

blue arrow indicates indicate the observer BEFORE he move closer to the mirror.

when the lower end of mirror is located half of his eye and his foot, then he can see his foot. And when he move closer to become red arrow, then light from his foot to the mirror can also reflect to his eye

so i think as he moves closer, he will not see less or more

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