*A smooth and highly polished reflecting surface is called a mirror.
(i) Plane Mirror A highly polished plane surface is called a plane mirror.
Different properties of image formed by plane mirror
Size of image = Size of object
Magnification == Unity
Distance of image == Distance of object
A plane mirror may form a virtual as well as real image.
A man may see his full image in a mirror of half height of man.
When two plane mirror are held at an angle θ, the number of images of an object placed between them is given as below
(a) n = [(360° / θ) – 1 ], where 360° / θ is an integer.
(b) n = integral part of 360° / θ, when 360° is not an integer.
[A plane mirror may form a real image, when the pencil of light incident on the mirror is convergent. Children, during their play form an image of sun as wall by a strip of plane mirror.]
Kaleidoscope and periscope employ the principle of image formation by plane mirror.
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 lengthand radius of curvature is given by
f=R/2
The power of a mirror is given as P = 1/f (metre) *
