Image formation in Convex Mirror and Lens

What is a mirror?

A mirror is an object used to reflect light. Mirrors are either plane or spherical. The plane mirror has a flat reflecting surface and the spherical mirror has a curved reflecting surface. The reflected rays follow the laws of reflection. The two laws of reflection are

I Law: The angle of incidence of the light ray is equal to the angle of reflection.

II Law: The incident ray, reflected ray and the normal is in the same plane.

Spherical Mirror

The spherical mirror is a mirror which looks like a curved shape sliced out of a sphere. They are of two types, concave mirrors and convex mirrors. If the reflecting surface is the inner surface it is called a concave mirror. If the bulged out surface is the reflecting surface then it is called a convex mirror.  

Some of the Important Terms Related to Image Formation of Mirror

Pole (P): It is the geometrical centre of the spherical surface of the mirror. It is usually represented by the letter P.

Centre of Curvature (C): The centre of the original sphere of which the spherical mirror is part is called the centre of curvature. 

Radius of curvature(R): Radius of the sphere of which the spherical mirror is a part.

Principal Axis: It is the straight line passing through the pole and the centre of curvature.

Focal point: The point at which rays meet after reflection or the point from which diverging rays appear to proceed is called Focal point.

Image Formation In Convex Mirror

A convex mirror is also called the diverging mirror due to the fact that incident light originating from the same point will reflect off the mirror surface and it diverges. Ray diagrams are a useful tool in determining the image formation in convex mirror. The convex mirror only produces a virtual image. A virtual image is an image that cannot be obtained on a screen. The image is located behind the convex mirror, it is an upright image smaller than the actual object. There are two rules for reflection of light for a convex mirror.

  1. Any incident light parallel to the principal axis will reflect in such a manner that its extension appears to pass through the focal point.
  2. An incident ray travelling towards the convex mirror such that its extension appears to pass through the focal point then the reflected light will travel parallel to the principal axis.


A lens is a piece of glass or transparent material that will converge or diverge the light rays by means of refraction. The lens follows the laws of refraction. The laws of refraction are

I Law: The incident ray, normal and the refracted ray all lie in the same plane.

II Law: The ratio of the sine of the angle of incidence to the sine of the angle of refraction will be a constant for the given pair of media and light of given wavelength.

The lens is of two types, concave lens and concave lens. The convex lens is thick in the middle and thinner in the edges. A concave lens is flat in the middle and thicker at the edges.