I know that plane mirrors form a virtual image and virtual images are formed behind the mirror. But I am unable to understand why. I know that the object's light gets reflected back to it through mirrors so when you look at the mirror, you are able to see yourself and your position with respect to the mirror. But to me, it just seems a copy of the real world behind the mirror. I am unable to understand why we perceive the image like this. I feel really confused right now. It could be that I have grasped something wrong. I would appreciate any intuition behind the images formed by mirrors.
Think about the formation of a real image of a point.
Light diverges from the point object and some of it passes through the eye's optical which converges the light as a point (real image) on the retina of the eye.
You "see" the point object.
Now consider the point object on front of a plane mirror.
Again light diverges from the point object and is reflected from the plane mirror and continues to diverge and some of it passes through the eye's optical which converges the light as a point (real image) on the retina of the eye. You "see" the point object.
As far as the eye is concerned the two situations are the same in terms of the formation of a real image on the retina of the eye and the eye "sees" a point object.
In the second case the diverging light as it reaches the eye appears to come from a point (which is called the virtual image of the object formed by the mirror) as far behind the mirror as the object is in front of the mirror with the line joining the virtual image and the object perpendicular to the mirror.
If one back produces the rays of light just before they intersect at what is called the virtual image.
So you may ask, So why do perceive them at some distance away from the mirror?
That is where the back produced rays cross and it is where the rays appear to come from.
The rays entering the eye from the virtual image behind a mirror are exactly the same as the rays entering the eye from a real object at that same location without the mirror being present.
The image as seen in the mirror (bottom diagram) is the same distance from the lens as the object in the top diagram.