Light: Spherical Mirror

Mirrors having curved reflecting surface are called spherical mirrors. A spherical mirror is a part of a sphere.

These notes are based on the chapter Light: Reflection and Refraction from class 10 science NCERT book and CBSE syllabus

Types of Spherical Mirror

Concave Mirror: Spherical mirror with reflecting surface curved inwards is called concave mirror.

Convex Mirror: Spherical mirror with reflecting surface curved outwards is called convex mirror.

You can take a shiny bowl to understand the difference between concave and convex mirror. The inner side of the bowl is a concave mirror, while the outer side is a convex mirror.

Important terms related to spherical mirror

Pole

The centre of reflecting surface of a spherical mirror is known as Pole. Pole lies on the surface of spherical mirror. Pole is generally represented by ‘P’.

Centre of Curvature

The centre of sphere, of which the reflecting surface of a spherical mirror is a part, is called the centre of curvature of the spherical mirror. Centre of curvature is not a part of spherical mirror rather it lies outside the mirror. Centre of curvature is denoted by letter C.

In the case of concave mirror centre of curvature lies in front of the reflecting surface. On the other hand, centre of curvature lies behind the reflecting surface in the case of convex mirror.

Radius of Curvature

The radius of sphere, of which the reflecting surface of a spherical mirror is a part, is called the Radius of Curvature of the spherical mirror. The radius of curvature of a spherical mirror is denoted by letter R.

Similar to centre of curvature, radius of curvature lies in front of concave mirror and lies behind the convex mirror and is not a part of the mirror as it lies outside the mirror.

Aperture

The diameter of reflecting surface of a spherical mirror is called aperture.

Principal Axis

Imaginary line passing through the centre of curvature and pole of a spherical mirror is called the Principal Axis.

Focus or Principal Focus

Point on principal axis at which parallel rays (coming from infinity) converge after reflection is called the Focus or Principal Focus of the spherical mirror. Focus is represented by letter F.

Converging Mirror

Fig: Converging Mirror

In the case of a concave mirror, parallel rays (coming from infinity) converge after reflection in front of the mirror. Thus, the focus lies in front of a concave mirror.

Diverging Mirror

Fig: Diverging Mirror

In the case of a convex mirror, parallel rays (coming from infinity) appear to be diverging from behind the mirror. Thus, the focus lies behind the convex mirror.

Focal length

The distance from pole to focus is called focal length. Focal length is denoted by letter f. Focal length is equal to half of the radius of curvature.

Or, `f=R/2`

Or, `R=2f`