Chapter 9 Ray Optics and instrument Class12 Physic, Chapter9 Ray Optics & Instrument | Page 5

If keeping an object fixed a plane mirror is rotated in its plane by an angle θ , then the reflected ray rotates in the same direction by an angle 2 θ .
Focal length as well as radius of curvature of a plane mirror is infinity . Power of a plane mirror is zero .
An image formed by a plane mirror is virtual , erect , laterally inverted , of same size as that of object and at the same distance as the object from the mirror .
( ii ) Spherical Mirror A highly polished curved surface whose reflecting surface is a cut part of a hollows at glass sphere is called a spherical mirror . Spherical mirrors are of two types
( a ) Concave Mirror A spherical mirror whose bent in surface is reflecting surface , is called a concave mirror .
( b ) Convex Mirror A spherical mirror whose bulging out surface is reflecting surface , is called a convex mirror .
Some Terms Related to Spherical Mirrors are Given Below
( i ) Centre of Curvature It is the centre of the sphere of which the mirror or lens is a part .
( ii ) Radius of Curvature ( R ) The radius of the hollow sphere of which the mirror is a part , is called radius of curvature .
( iii ) Pole The central point of the spherical mirror is called its pole ( P ).
( iv ) Focus When a parallel beam of light rays is incident on a spherical mirror , then after reflection it meets or appears to meet at a point on principal axis , which is called focus of the spherical mirror .
( v ) Focal Length The distance between the pole and focus is called focal length ( f ). Relation between focal length and radius of curvature is given by