# Focal length of Concave and Convex lens

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Introduction

Focal length of Concave and Convex lens

Objective

In this experiment, we are going to use three different methods and one method to find the focal length of an unknown-focal length of convex lens and concave lens.

## Apparatus

2 meter rules, a light box, a white-coloured screen, a D.C. power supply, 2 lens holders, a plane mirror, a transparent object F, two-unknown focal length of convex and concave lens.

## Theory

Since the convex and concave lenses are made of glasses, the focal lengths are actually the convergent point of all refracted light sources. So, the main theory that we are applying is the refraction theory and also the Lensmaker's equation.

is an equation which shows the relationship between the focal length with the object and image distances. With mathematical simplification, the Lensmaker's equation can be applied as follow .

Procedure

Type A is a convex lens with shorter diameter

Middle

First, put a light source with a transparent object F in front of a convex lens.

Then adjust the distance between the lens and the light source and mark down the distance as D when you can form a sharp image on the screen allocated right beside the light source. The length D is the focal length

METHOD III

By putting the light source in front of the lens and then adjust the distance between the lens and light source and the lens and the screen. Obtain a sharp image and mark down the distance of U and V.

Then plot a graph with U and V to find out the focal length of the lens by applying the equation.

Concave lens

Place the light source in front of the convex lens which is placed between the concave lens and the light source.

Conclusion

Actually, whenever we test the focal length of the lenses we need to consider the degree of the curve of the lens. Since some light rays may refracted near the edge of the lenses and the refracted ray may thus get another focal length and thus resulting a unclear image.

On the other hand the nodal inside the lenses is another interesting topic that students should know. Although we cannot apply the knowledge of it in this experiment, I think most of us should know more about it. It is because this is another factor that affecting the degree of refraction of light rays in lenses.

Lastly, since light ray consist of 7different colours,and what we need to know that the distortion of the rays may happen. So we need to make the total distance of the experiment much smaller.

This student written piece of work is one of many that can be found in our GCSE Waves section.

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