The figure below shows a point source of light $S$,a convex lens $L$,and a plane mirror $M$. The three are placed such that rays of light from $S$ return to it after reflection from $M$. $(i)$ What is the distance $OS$ called? $(ii)$ To which point (left of $S$,on $S$,or right of $S$) will the rays return if $M$ is moved to the left and brought in contact with $L$?

(N/A) $(i)$ The distance $OS$ is called the focal length of the lens.
$(ii)$ The rays will still return to the same point $S$. When the mirror $M$ is placed in contact with the convex lens $L$,the rays from the point source $S$ (placed at the focus) pass through the lens and become parallel to the principal axis. These parallel rays strike the plane mirror $M$ normally and reflect back along the same path. After passing through the lens again,they converge at the point source $S$ itself.