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# Measurement of the focal lengths of a concave lens and a convex mirror

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Introduction

Kwun Tong Government Secondary School

Advanced Level Physics (TAS)

Experiment Report

Experiment No.:              3

## concave lens and a convex mirror

Date of Experiment:           2006/11/28

Mark:

 Sch. Code 2114 TAS Group 15 Name: Chan Man Lok Class: S.6C Class No.: 16

Objective:

1. To measure the focal length of a concave lens with the help of an auxiliary convex lens and the lens formula
1. To measure the focal length of a convex mirror with the help of an auxiliary convex lens

Apparatus:

## Instrument

Convex lens

x 1

Concave lens

x 1

Convex mirror

x 1

Metre rule

x 1

White screen

x 1

Ray box with a fine gauze grid

x 1

Lens holder

x 2

Theory:

Measurement of the focal length of a concave lens using an auxiliary convex lens:

1. Position the screen to catch the real image formed by the auxiliary convex lens
1. Add the concave lens between the convex lens and the screen and reposition the screen to catch the image of fine grid again
1. Calculate the focal length of the concave lens f by using the lens formula:

By using real-is-positive convention, u will be taken as negative.

Middle

I forms coincide with the object O due to the fact that the reflected beam reflects back along its original path when the convergent beam is incident normally on the mirror. The distance between C and the pole of the mirror P is then the radius of curvature r.

Procedure:

Estimating the focal length of the convex lens f :

1. The focal length of the convex lens f was estimated by using a distant object.

A.  Measurement of the focal length of a concave lens using an auxiliary convex lens:

1. The convex lens was placed about 2f from the illuminated fine gauze grid and a sharp image was tried to be caught on the white screen.
1. The concave lens was then placed about half way between the convex lens and the screen, The distance between the concave lens and the screen u was measured. After that, The screen was moved away from the lens until a sharp image of the fine gauze grid was again obtained. The new distance between the concave lens and the screen v was measured.

Conclusion

1. The sharpness of the images were observed by the human naked eyes

Since the sharpest image obtained was difficult to be observed by human naked eyes, there was a range of distance where could obtain relatively sharper images. As a result, the distances obtained were inaccurate and hence the results become inaccurate. To improve the experiment, the images should be observed by several observers instead of one.

1. The wearing of the metre rule

The common zero error arises from using the metre rule from one end, which was worn. It is due to the deviation from “0” value at the “0” mark. So using the centre of the rule instead was needed.

1. Parallax error

A human error called parallax error arises when observers looked at the graduated scale like A and C. To reduce the error, Looking at the graduated scale right from above, like B to obtain the reading was necessary.

Conclusion:

Through the experiment, the focal length of the concave lens was found to be 17.25cm. The focal length of the convex mirror was found to be 11.04cm.

Reference:

① : Picture from http://www.jjjtrain.com/vms/Media/glossary_p/parallax.gif

S.6 Physics TAS Experiment 3            Chan Man Lok 6C (16)              P.

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