Why 0.3m From Lens Is Optimal Focus?

[phantomvommand]

Homework Statement

A lens has a focal length of 0.3m. A plane mirror is placed 0.2m behind the lens. Where must an object be placed, for its image to coincide with its position?

Relevant Equations

Ray diagrams
thin lens formula

Apparently, it must be placed at the focus (0.3m away from the lens). Why is this so?

 

[haruspex]

Homework Statement:: A lens has a focal length of 0.3m. A plane mirror is placed 0.2m behind the lens. Where must an object be placed, for its image to coincide with its position?
Relevant Equations:: Ray diagrams
thin lens formula

Apparently, it must be placed at the focus (0.3m away from the lens). Why is this so?

If the object is at the focus, what can you say about the rays from a given point on the object after they have passed through the lens?
Follow their path on to the mirror and back through the lens.

 

[phantomvommand]

If the object is at the focus, what can you say about the rays from a given point on the object after they have passed through the lens?
Follow their path on to the mirror and back through the lens.

Won't the rays that emerge from the lens all be parallel to each other? But the rays hit the mirror at different points. How will the reflected paths then reconverge to form the image?

 

[phantomvommand]

If the object is at the focus, what can you say about the rays from a given point on the object after they have passed through the lens?
Follow their path on to the mirror and back through the lens.

I see what you mean. Thank you very much