Why is the image formed next to the object?

[Wasif Shah]

http://imgur.com/a/nFJ0a

According to this passage the image of an object after being reflected by a plane mirror is formed right next to the initial object position. However if you see the light rays, they come back exactly the same path after striking the mirror, then why doesn't the image also forms right on top of the illuminated object (cross wire), and instead forms on the side?

 

[BvU]

Your intuition is completely correct if the planes of lens and mirror are exactly parallel AND the lens axis goes through the center of the object.
But to perform this determination of the focal length, it is very practical if the image is not exactly on top of the object, but a bit to a side.
This can be achieved by shifting the lens a little bit sideways.

You can convince yourself by drawing a top view of the image formation with an off-axis object.

 

[Wasif Shah]

Nope I don't get it :/ . Esp the last part when you said 'it is very practical if the image is not exactly on top of the object, but a bit to a side'

How is the image even produced next to it? I feel it should exactly overlap the object. :/

 

[pixel]

Nope I don't get it :/ . Esp the last part when you said 'it is very practical if the image is not exactly on top of the object, but a bit to a side'

How is the image even produced next to it? I feel it should exactly overlap the object. :/

Suppose you took the layout as shown in your diagram and rotated the mirror slightly about a vertical axis. Then the parallel rays that leave the mirror would enter the lens at a slight angle with respect to the parallel rays leaving the lens. As a result, the image will form a little to the side of the light. Typically you would hold a screen (such as a sheet of paper) next to the light and move the lens toward or away from the light until a sharp image forms right next to the light. Then you know the lens is a focal length away from the light.

 

[Wasif Shah]

Thankyou soo much I get it <3 :)