principal focus and focal length of sherical mirror.
Answers
Answer:
Focal Length of Spherical Mirrors
November 29, 2016 by Sastry CBSE
Focal Length of Spherical Mirrors
Spherical mirror
(a) Definition: A mirror whose surface is cut out of a spherical shell, is called a spherical mirror.
(b) Types: There are two types of spherical mirrors:
Concave mirror: Its outer convex surface is polished and inner concave surface reflects.
Convex mirror: Its inner concave surface is polished and outer convex surface reflects.
Terms associated with spherical mirrors

Aperture. The diameter of the circular rim of the mirror, is called the aperture of the mirror. Hence, aperture is a part of mirror, through which rays of light entered.
In diagram MM’ is the aperture of the mirror.
Pole. The centre of the spherical surface of the mirror, is called the pole of the mirror. It lies on the surface. It is denoted by P.
In diagram, P is the pole of the mirror.
Centre of curvature. The centre of the spherical shell, of which the mirror is a section, is called the centre of curvature of the mirror. It lies outside the surface. Every point on the mirror surface lies at same distance from it. It is denoted by C.
In diagram, C is the centre of curvature of the mirror.
Principal axis. The straight line passing through the pole P and the centre of curvature C of the mirror, is called principal axis of the mirror.
Principal focus It is a point on the principal axis of the mirror, such that the rays incident on the mirror parallel to the principal axis after reflection, actually meet at this point (in case of a concave mirror) or appear to come from it (in case of a convex mirror). It is denoted by F.
In diagram, F is the principal focus of the mirror.
Radius of curvature. The distance between the pole and the centre of curvature of the mirror, is called the radius of curvature of the mirror. It is equal to the radius of the spherical shell, of which the mirror is a section.
In diagram, PC is the radius of curvature of the mirror. It is represented by the symbol R.
Focal length. The distance between the pole and the principal focus of the mirror, is called the focal length of the mirror.
In diagram, PF is the focal length of the mirror. It is represented by the symbol f.
For mirrors of small aperture, f=R/2.
Principal section. A section of the spherical mirror cut by a plane passing through its centre of curvature and the pole, is called a principal section of the mirror. It contains the principal axis.
In diagram, MPM’ is the principal section of the mirror cut by the plane of the book page.
principle focus :- a point where all light rays meet after reflection
focal length :- distance between focus and pole of mirror
hope it helps you buddy