If p be the object size and q be the image size. Both concave and convex spherical mirrors asymptote to plane mirrors in the limit in which their radii of curvature 
tend to infinity. In other words, a plane mirror can be treated as either a concave or a convex mirror for which 
. Now, if , then 
, so 
, and yields
tend to infinity. In other words, a plane mirror can be treated as either a concave or a convex mirror for which . Now, if , then , so , and yields PLEASE EXPLAIN THE SYMBOLS USED HERE..
When radius of curvature tends to a extremely large value ie , then focal length
As
1/∞ → 0
So