# Focal Length Of Convex Lens | Science Experiment

## Introduction

In this science experiment, we will learn to determine the focal Length Of the Convex Lens.

## Lens

A lens comprises a transparent material bounded by two spherical surfaces of the same or different radii.
The curved surface of the lens is of two types, concave and convex.

## Types Of Lenses

1. CONCAVE LENS: Both refracting surfaces of this lens are convex. The concave lens is thicker in the middle and thinner at the edge.
2. CONVEX LENS: Both refracting surfaces of this lens are concave. The Conves lens is thinner in the middle and thicker at the edge.

## Important Terms On Lense

1. OPTICAL CENTRE – The centre of the mirror is located on the principal axis through which the incident ray passes without much deviation.
2. CENTRE OF CURVATURE – It is the centre of a sphere of which the spherical lens is a part. Lens has two refracting surfaces C1 and C2.
3. RADIUS OF CURVATURE – The radius of the sphere of which the spherical lens is a part. Lens has two refracting surfaces R1 and R2.
4. PRINCIPAL AXIS – The line joining the centre of curvature of two refracting surfaces is called the principal axis.
5. PRINCIPAL FOCUS
a) Convex lens – The point where the parallel rays of the light meet after reflection is called the principal focus of the convex lens.
b) Concave lens – The point where the parallel rays of light appear to meet after reflection is called the principal focus of the concave lens.
6. FOCAL LENGTH – The distance between the principal focus and the optical centre is called the lens’s focal length.

## Lens Formula

The relation between the object distance u, image distance v, and focal length is described by the formula ;

1/f = 1/v – 1/u

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

## Important Points Of Convex Lens

1. Parallel light rays coming from a distance source will meet at the focus after reflection.
2. Light rays coming from focus pass parallel to the principal axis after reflection.
3. Light rays coming from the optical centre of the lens 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. The image formed by a convex lens based on the position of the image.
6. Images formed can be real and inverted, virtual and erect, magnified or enlarged.
7. The linear magnification of a convex lens can be one, less than one, and greater than one.
8. Magnification of convex lens is positive for virtual images and negative for real images.
9. Magnification of the concave lens is always positive.
10. A convex lens is also called a convergent lens because it converges the light rays at one point.

1. Camera,
2. Telescope,
3. Microscope
4. Eyeglasses
5. Projector

## Aim

To find the focal length of a convex lens by obtaining the image of a distant object.

## Apparatus Required

1. A lens holder,
2. A concave lense,
3. Meter scale,
4. White screen.

## Theory

1. The focal length of a lens 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. The light rays coming from a distant source, say tree, will be considered parallel light rays.
4. All the light rays will converge at the focus of the lens.
5. A real, inverted, highly diminished image will form when a screen is placed at focus.

## Procedure

1. Select a distant source of light, such as a tree.
2. Hold the lens with the help of a lens holder.
3. Adjust the convex lens such that light rays coming from the tree fall on its reflecting surface.
4. Obtain a well-defined image on a white screen by moving the concave mirror.
5. Now, measure the distance between the convex lens and 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, B Position of the lense, A cm Position of the screen, C cm Difference between lens and screen (A – C)cm Focal length,f cm 1. f1 = …….cm 2. f2 = …….cm 3. f3 = …….cm

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

= ………….cm.

## Result

The value of focal length of convex lense = ………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. The Meter scale should be correctly placed between the white screen and the concave mirror.
5. The lense must be thoroughly clean.
Note:- This experiment is not helpful for concave lenses because concave lenses form virtual and erect images which can not be obtained on a white screen.

## Conclusion

In this way, we have learned to determine the focal length of a convex lens 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 convex lens by obtaining the image of a distant object.

Q.2 What is the lense formula?
ANS.  1/f = 1/v – 1/u

Q.3 What is the nature of the image formed in this experiment?
ANS. Real, inverted, and much smaller than the object.

Q.4 When does a convex lens form a diminished image?
ANS. When the object is placed between infinity and the optical centre.

Q.5 Which phenomena of light lens shows?
ANS. Refraction.

Q.6 Which phenomena of the light mirror shows?
ANS. Reflection

Q.7 What is the difference between reflection and refraction?
ANS. When the light reflects back in the same medium, it’s called reflection of light. When the light passes from one medium to another with the deflection in its path, it is called the refraction of light.

Q.8 Which lens is called a convergent lens?
ANS. Convex lens.

Q.9 What is the other name for a concave lens?
ANS. Divergent lense.

Q.10 Why convex lenses are called convergent lense?
ANS. Because a convex lens converges all light rays at one point.