# Focal Length Of Concave Mirror | Science Experiment

## Introduction

In this science experiment, we will learn to determine the focal length of a concave mirror.

## Spherical Mirror

Spherical mirrors are a type of curved mirror whose one reflecting surface is spherical, and another surface is coated with mercury and paint to protect from damage.

There are two types of spherical mirrors;

1. CONCAVE MIRROR – This spherical mirror has its reflecting surface curved inward, i.e. reflecting surface looks towards the centre of the sphere.

2. CONVEX MIRROR – This spherical mirror has its reflecting surface curved outward, i.e. reflecting surface faces away from the centre of the sphere.

## Important Terms Used In Spherical Mirrors

1. PRINCIPAL AXIS – The line joining the centre of curvature and pole is known as the principal axis.

2. CENTRE OF CURVATURE – It is the centre of the spherical mirror from which the concave or convex mirror is formed.

3. RADIUS OF CURVATURE – It is the radius of the spherical mirror from which the

the mirror is formed.

4. POLE – The centre of the reflecting surface of a mirror is known as the pole of that mirror.

5. FOCUS – Focus is the point where the parallel light rays meet (concave mirror) or appears to meet (convex mirror).

6. APERTURE – The width of the reflecting surface of the mirror is called its aperture.

7. FOCAL LENGTH – The length between the pole and focus of a mirror is called the focal length, f. FOCAL LENGTH OF CONCAVE MIRROR FOCAL LENGTH OF CONVEX MIRROR

## Mirror Formula

The relation between u, v and f is given by:

1/u+q/v = 1/f

This formula is valid for both the mirror (concave or convex).

## Important Points Of Concave Mirror

1. Parallel light rays coming from a distant source passes from the focus after reflection.
2. Light rays coming from the focus pass parallel to the principal axis after reflection.
3. Light rays coming from the centre of curvature pass on the same path after reflection.
4. The light rays make an angle with the principal axis; after reflection, they will make an equal angle with the principal axis.
5. R = 2f is the relation between the radius of curvature and focus.
6. Real and inverted image is always formed in front of the concave mirror.
7. Virtual and erect images are always formed behind the mirror.
8. It converges the rays at one point.

## Uses Of Concave Mirror

1. Rear-view mirror,
4. Telescopes,
5. Flashlights etc.

## Aim

To find the focal length of a concave mirror by obtaining the image of a distant object.

## Apparatus Required

1. A mirror holder,
2. A concave mirror,
3. Meter scale.

## Theory

1. The focal length of a mirror is calculated by the formula 1/f = 1/v + 1/u
2. The nature and size of the image depend on the position of the object.
3. Objects should be placed on the left side of the mirror.
4. Image is formed on the right side of the mirror.
5. When a screen is placed at focus, a real and inverted image is formed.

## Procedure

1. Select a distant source of light, such as a tree.
2. Hold the mirror with the help of a mirror holder.
3. Adjust the concave mirror such that light coming from the tree falls on its reflecting surface.
4. Obtain a well-defined image on a white screen by moving a concave mirror.
5. Now, measure the distance between the concave mirror and the white screen with the help of a metre scale.
6. This is the measurement of focal length.
7. Repeat this experiment by changing distant sources with different distances. ## Observations And Calculations

 S.NO Distant object Position of the mirror, A cm Position of the screen, B cm Difference between mirror and screen (A – B)cm Focal length,f cm 1. f1 = …….cm 2. f2 = …….cm 3. f3 = …….cm

Mean value of focal length = ( f1 + f2 + f3 )/3

= ………….cm.

## Precautions

1. Measurement should be taken accurately.
2. The distant object must be clearly located.
3. The formed image should be sharp and well defined.
4. Meter scale should be correctly placed between the white screen and the concave mirror.

Note:- You may perform this experiment even outside the laboratory.

## Conclusion

In this way, we have learnt to determine the focal length of a concave mirror by obtaining the image of a distant object.

Q.1 What was the aim of our experiment?

ANS. To find the focal length of a concave mirror by obtaining the image of a distant object on the screen.

Q.2 What is the sign of an object in a concave mirror?

ANS. Negative sign.

Q.3 What is the relation between the radius of curvature and focal length.

ANS. R = 2F

Q.4 What is the mirror formula?

ANS.  1/f = 1/v + 1/u

Q.5 What is a spherical mirror?

ANS. Sphericals mirrors are a type of curved mirror whose one reflecting surface is spherical, and another surface is coated with mercury and paint to protect from damage.

Q.6 How many types of spherical mirrors are there?

ANS. Two. Convex mirror and concave mirror.

Q.7 What is the nature of an image formed by a concave mirror?

ANS. Real and inverted except for one position when the object is placed between pole and focus.

Q.8 What is the nature of an image formed when an object is kept between pole and focus?

ANS. Virtual and erect.

Q.9 What is the sign of an image formed in a concave mirror?

ANS. Positive sign.

Q.10 Describe the difference between a real image and a virtual image?

ANS. A real image can be obtained on a screen, while a virtual image can not be obtained on a screen.