1. The problem statement, all variables and given/known data In the attached picture , It has been explained in my book that " I the apeture of the mirror is small, the point P'( ponint of incidence) is very close to the point P( pole) ,Then PF(focal length)= P'F :. PF =Fc or PF=1/2R" I dont get it ! Plzzzz help!!