12th physics. NCERT notes


A smooth and highly polished reflecting surface is called a mirror.

Plane Mirror

A highly polished plane surface is called a plane mirror.

Different properties of image formed by plane mirror

  1. Size of image = Size of object
  2. Magnification == Unity
  3. Distance of image == Distance of object
  4. A plane mirror may form a virtual as well as real image.
  5. 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:

  1. n = [(360° / θ) – 1 ], where 360° / θ is an integer.
  2. 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.]

  • 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.

Spherical Mirror

A highly polished curved surface whose reflecting surface is a cut part of hollows at glass sphere is called a spherical mirror. Spherical mirrors are of two types:

Concave Mirror

A spherical mirror, whose bent in surface is reflecting surface, is called a concave mirror.

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:

  • Centre of Curvature: It is the centre of the sphere of which the mirror or lens is a part.
  • Radius of Curvature (R): The radius of the hollow sphere of which the mirror is a part, is called radius of curvature.
  • Pole: The central point of the spherical mirror is called its pole (P).
  • 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.
  • 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 f=R/2
    The power of a mirror is given as P = 1/f (meter)
  • Mirror formula: 1/f = 1/v + 1/u

where f = focal length of the mirror, u = distance of the object and v = distance of the image.

Newton’s formula for a concave mirror

where x1 and x2 are the distances of object and image from the focus.

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