In order to understand the image formation in spherical mirrors, we need to know about some of the terms related to them.

**Terms related to
Spherical Mirrors**

In
order to understand the image formation in spherical mirrors, we need to know
about some of the terms related to them.

**Center of Curvature**

It
is the center of the sphere from which the mirror is made. It is denoted by the
letter C in the ray diagrams. (A ray diagram represents the formation of an
image by the spherical mirror. You will study about them in the higher
classes).

**Pole**

It
is the geometric centre of the spherical mirror. It is denoted by the letter **P**.

**Radius of Curvature**

It
is the distance between the center of the sphere and the vertex. It is shown by
the letter **R** in ray diagrams (The
vertex is the point on the mirror’s surface where the principal axis meets the
mirror. It is also called as ‘**pole**’).

**Principal Axis**

The
line joining the pole of the mirror and its center of curvature is called
principal axis.

**Focus**

When
a beam of light is incident on a spherical mirror, the reflected rays converge
(concave mirror) at or appear to diverge from (convex mirror) a point on the
principal axis. This point is called the ‘focus’ or ‘principal focus’. It is
also known as the focal point. It is denoted by the letter F in ray diagrams.

**Focal length**

The
distance between the pole and the principal focus is called focal length (f) of
a spherical mirror.

There
is a relation between the focal length of a spherical mirror and its radius of
curvature. The focal length is half of the radius of curvature.

Focal
length = Radius of curvature / 2.

**Problem 1**

The radius of curvature of a spherical mirror is 20 cm. Find its
focal length.

**Solution**

Radius of curvature =
20 cm

Focal length (f) =
Radius of curvature / 2

= R/2 = 20/2 = 10 cm

**Problem 2**

Focal length of a spherical mirror is 7 cm. What is its radius
of curvature?

**Solution**

Radius of curvature (R) = 2 × Focal length

= 2 × 7 = 14 cm

Tags : Light | Chapter 3 | 8th Science , 8th Science : Chapter 3 : Light

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