# 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 .

Middle

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. Then adjust the distance between the convex lens and the light source and the concave lens and the convex lens to obtain a sharp image on the screen. Mark the distance between the two lens when and sharp image is formed on the screen as S. Next minus F with S to get the focal length of the concave lens. Data analysis After the data analysis we obtain several stat. of the focal length. Convex lens A: 22.8cm was obtain from method I 22.3cm from method II 19-23.4cm from method III Convex lens B: 5.1 cm was obtain from

Conclusion

It is because the image that we obtained is slightly tolerated. 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. ?? ?? ?? ??

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