Question
What is difference between focus and principal focus?
Solution
Principal focus is a point on the principal axis where the rays which are incident parallel to the principal axis meet after reflection.
Principal focus is a point on focal plane and is located on the principal axis.
Focus is a point where rays parallel to each other meet. It is not necessary for the rays to be parallel to the principal axis.
These are infinite foci, all lying in a plane called focal plane.
