Why object size in a mirror is changing?

[Tinamuline]

As we all know the image formed by the mirror must be the same size as the object, however, when the object moves away from the mirror, the size of the image becomes smaller. I would like to know why this happens since the size of the object remains the same. The further the object moves away from the mirror, the smaller the size of the image is. Using this logic, why does the object become a point size when it is far away from us. Please let me know if you have the answer. Thank you.

 

[jayc]

Re: Object size

Let's use a simple example. Imagine you are standing 1 foot directly in front of a mirror. How far away from you does the reflected image appear to you? Just look at the position of the feet. Your own feet are 1 foot away from the mirror, and the reflection of you appears 1 foot away from the mirror, so actually, the "distance" between your reflection and you is twice the distance between you and the mirror. So as you walk farther away from the mirror, your reflection does so as well, so the total distance between you and your reflection will always be twice the distance between you and the mirror itself. Therefore, if you walk very far away from the mirror, say 100 feet, then the reflection will appear to be 200 feet away, which will probably look like a small point in the mirror.

Of course, this is neglecting some math due to the fact that you are not a point particle and your eye level is roughly a point whereas your body is not. However, this assertion is approximately true.

 

[Tinamuline]

Re: Re:Object size

Thank you for reply. However I still don't understand why objects become point size when they are far far away from us. Please help.

 

[maxwelleb]

Re: Object size

your eye "sees" the object under some angle;
when object (or you or both) moves away this angle changes;
as object goes more far this angle approaches zero;
this is the angle a point produces;

 

[Tinamuline]

Re: Object size

Thank you. It really helps.