# The Radius of Curvature of a Spherical Mirror is 20cm. What is Its Focal Length?

By BYJU'S Exam Prep

Updated on: September 25th, 2023

The focal length is 10 cm.

Step 1: Given that

The radius of curvature of a spherical mirror, that is r=20cm

The focal length of a spherical mirror, that is f=?

Step 2: The formula to find focal length is mentioned below.

A particular mirror’s focal length is equal to half of its radius of curvature.

f=r/2

Step 3: calculation using the formula

f=r/2

Put r = 20 we get

f=20/2

f=10cm

Therefore, the focal length of the given mirror is 10 cm.

### History of Spherical Mirror –

It is plausible that early humans used aquatic bodies of water as mirrors. The first manufactured mirrors that are known to have existed were polished stone artefacts. Around 500 AD, the Chinese people first used silver and mercury amalgams to create mirrors. Curved polished iron mirrors were employed by Claudius Ptolemy in a variety of research.

Focal Length –

1. The focal length of a mirror (or lens) is the separation between its pole and focal point. It can also be measured as the distance between its optical centre and pole.
2. The point at which a parallel beam of light is focused after reflection in the mirror is known as the focal point (F) of a mirror.
3. For a convex lens and convex mirror, the focal length is assumed to be positive.
4. For a concave lens and a concave mirror, it is regarded negatively.
5. The symbol for it is f.

Summary:-

## The Radius of Curvature of a Spherical Mirror is 20cm. What is Its Focal Length?

The radius of curvature of a spherical mirror is 20cm. The focal length is 10 cm.

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