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I know this seems very basic. I was watching a video on reflection by plane mirrors and the teacher said that the formation of image happens inside the mirror and not on the backside.

But, suppose I am standing a metre away from the mirror, how does the image form one metre inside the mirror?

We say that distant of object from mirror is the same as the distance of plane from the mirror(in a plane mirror), but doesn't that imply that I need to have a one metre thick mirror to see the image of an object standing one metre away from the mirror?

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  • $\begingroup$ "formation of image happens inside the mirror and not on the backside" That is poorly worded. Poorly worded presentations can communicate incorrect ideas, and that is just what has happened here. I would not trust presentations by that author. $\endgroup$
    – garyp
    Feb 7, 2020 at 11:23

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An image is said to be formed when the light rays from an object converge (or appear to converge) at another point. The position of this point decides if the image is real or virtual. In case of the plane mirror, the light rays simply coincided at a distance of $1\text m$ behind the mirror.

I am not sure what you mean in your second question. A mirror does not have any thickness as such, it is just a surface which reflects all inbound rays of light.

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Plane mirror forms a virtual image of the object, virtual image never forms (form means light rays need to converge at a point) but the reflected rays from the mirror seems to be coming from some point inside the mirror and hence we say an image is formed by the mirror (which is basically sloppy).

See this diagram enter image description here notice that the dotted lines do not EXIST they are just assumed (the purple line represents the mirror) . By the laws of reflection, I have marked the angles. I have assumed in this case that image formed by the plane mirror is of same size as the object, so by some simple maths $$ \tan \theta = \frac{L}{d} ~\textrm{and}~ \tan \theta = \frac{L}{d'}$$ $$ \textrm{therefore}~ \frac{L}{d} = \frac{L}{d'}$$ $$ \implies d = d'$$ So, we have proved that image will be formed (again be careful by the word formed) inside the mirror as much as the object is in front of it. But you must see this result is purely mathematical and has come from assumption that dotted rays exists (that the reflected rays seems to be coming from some point inside the mirror). You can see that the thickness of mirror has no role to play in all these formulations.

Hope this helps!

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