Determination Of Focal Length Of Concave Mirror And Convex Lens

In this article, we will discuss about focal length of convex lens, focal length of concave mirror, what focal length of a concave mirror is, focal length of concave lens, focal length of convex mirror, lens focal length, what the focal length of a concave mirror is, focal length of spherical mirror, what is a concave mirror of focal length f, how to find focal length, focal length of mirror, what is the focal length of a concave mirror, convex lens focal length sign and concave mirror focal length sign.

JEE Main 2025: Physics Formula | Study Materials | High Scoring Topics | Preparation Guide

JEE Main 2025: Syllabus | Sample Papers | Mock Tests | PYQs | Study Plan 100 Days

NEET 2025: Syllabus | High Scoring Topics | PYQs

This Story also Contains

1. Introduction
2. Types of Mirrors
3. Types of Spherical Mirror
4. There are two types of spherical mirrors:-
5. Terminology of the spherical mirrors
6. How to find the focal length of a concave mirror?
7. Types of lens
8. How to find the focal length of a convex lens?

Introduction

What is the focal length of a convex lens and a concave mirror?

First of all let us understand Concave Mirror and Convex Lens.

Let us define Mirror. A mirror is a reflecting surface which has a coating on one of its surfaces.

Also read -

• NCERT Solutions for Class 11 Physics
• NCERT Solutions for Class 12 Physics
• NCERT Solutions for All Subjects

Types of Mirrors

There are two types of mirrors i.e. Plane mirror and Spherical mirror.

Plane Mirror:- It is defined as a flat, smooth surface that reflects most of the light rays falling on it.

Spherical Mirror:- A spherical mirror is a part of a hollow sphere with coating on one of its surfaces.

Types of Spherical Mirror

There are two types of spherical mirrors:-

1. Concave mirror:- It is a type of spherical mirror whose reflecting surface is bulged inward and the outward bulged surface is polished. It is also known as a converging mirror as it converges the light rays at one point.

2. Convex mirror:- It is a type of spherical mirror whose reflecting surface is bulged outward and the inward bulged surface is polished. It is also known as a diverging mirror as it diverges the light rays.

Terminology of the spherical mirrors

1. Pole:- We can define pole as the midpoint of the reflecting surface of a spherical mirror. We always find it lying on the surface of the mirror. It is represented by P.

2. Centre of Curvature:- The center of the sphere of which the reflecting surface of a spherical mirror forms a part is called the center of curvature. It is represented by C.

3. Radius of Curvature:- The radius of the sphere of which the reflecting surface of a spherical mirror forms a part is called the radius of curvature of the mirror. It is the distance of the center of curvature from the pole of the mirror. It is represented by R.

4. Principle Axis :- We can define it as a straight line passing through the pole and center of curvature of a spherical mirror .

5. Aperture:- The diameter of the reflecting surface of the spherical mirror can be defined as the aperture of the mirror. The aperture of the mirror should be smaller than its radius of curvature.

6. Focus:- We can define it as the midpoint of radius of curvature of a spherical mirror.

7. Focal Length:- We can define focal length of a concave mirror as the distance between the pole and the focus of a spherical mirror. It is represented by f. Mathematically, it is represented by,

Now, we will find the focal length of a concave mirror.

How to find the focal length of a concave mirror?

For this we need to hold a concave mirror in our hand and face its reflecting surface towards the Sun. We will try to direct the reflected light by the mirror onto a sheet of paper and hold it closer to the mirror. Now, we will move the sheet gradually until we find a bright, sharp spot of light on the sheet of the paper. At this position, we will measure the distance of the spot from the mirror. This length is the focal length of the concave mirror. If we keep the paper there for some time, we will observe that the paper begins to burn and produces smoke. We will notice that it may even catch fire after a while. In fact, the concentration of the sunlight at a point generates heat which causes the paper to burn. Now let us also recall the lenses and define a lens.

Lens:- We define a lens as a transparent material bounded by two surfaces, of which at least one surface is spherical.

Types of lens

There are two types of lens:-

1. Concave Lens:- The lens that is bounded by two spherical surfaces which are curved inwards is called the concave lens.

2. Convex Lens:- The lens that is bounded by two spherical surfaces which are bulging outwards is called a convex lens.

Terminology of the spherical lens

1. Optical Centre:- The central point of a lens is called its optical center. It is represented by O.

2. Center of Curvature:- We know this fact that a lens is bounded by two spherical surfaces. These surfaces are the part of a sphere. The centers of these two spheres are known as centers of curvature of the lens.

3. Principal Axis:- We can define it as a straight line passing through the center of curvature of the two surfaces of a lens.

4. Aperture:- We can define it as the effective diameter of the circular outline of a spherical lens.

5. Principal Focus:- When the rays of light parallel to the principal axis incident on a convex lens, the rays converge to a point on the principal axis after refraction. This point is called principal focus. It is represented by F.

6. Focal Length:- We can define the focal length of a Convex Lens as the distance of principal focus from the optical center of a lens. It is represented by f. Now, we will find the focal length of a Convex Lens.

Also Read:

• NCERT solutions for Class 12 Physics Chapter 9 Ray Optics and Optical Instruments
• NCERT Exemplar Class 12 Physics Solutions Chapter 9 Ray Optics and Optical Instruments
• NCERT notes Class 12 Physics Chapter 9 Ray Optics and Optical Instruments

How to find the focal length of a convex lens?

We need to follow the same steps as we followed for finding the focal length in case of a concave mirror. For that first of all we need to take a convex lens and hold it in our hand directing it towards the Sun. We know that the Sun is at infinity, so it will form the image at the focus of the lens. After that we focus the light from the Sun on a sheet of paper which will act like a screen. Now, we need to move the sheet of paper back and forth gradually until we find a bright and sharp image of the Sun on the sheet. The point on the sheet where a bright and sharp image of a Sun is obtained is the focus of the convex lens. The light rays from the Sun constitute parallel rays of light. So, these light rays were converged by the lens at the sharp bright spot formed on the paper. Also, we can notice that the distance between the position of the lens and the position of the image of the Sun gives us the approximate focal length of the lens. If we hold the paper and the lens in the same position for a while, we will observe that the paper begins to burn and produces smoke. It may even catch fire after a while. In fact, the concentration of the sunlight at a point generates heat which causes the paper to burn.

Also check-

• NCERT Exemplar Class 11th Physics Solutions
• NCERT Exemplar Class 12th Physics Solutions
• NCERT Exemplar Solutions for All Subjects

NCERT Physics Notes:

• NCERT Notes Class 11th Physics
• NCERT Notes Class 12th Physics
• NCERT Notes For All Subjects