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# Find the focal length of a given convex lens.

Extracts from this document...

Introduction

Experiment no. 9

Aim: -

To find the focal length of a given convex lens.

Apparatus: -

Convex lens                                           Metre rule

Screen                                           Candle

Matches                                   Wooden blocks

Theory:-

In this experiment the focal length of a lens is found out. The focal lens of a lens is the distance from the principal focus to the optical centre. It is the point where the image comes into focus when seen though the lens. The focal length of a lens depends on the curvature of each surface. So the more highly curved the surface, the shorter is the focal length and visa-versa. In this experiment the lens is first placed at L1, between the screen and the illuminated object. When the illuminated object comes in focus on the screen the distance of L1

Middle

The independent variable is the distance d, the distance between the screen and the illuminated object.

The controlled variable is the intensity of the illuminated object and the focal length of the lens

Procedure

1. Place the illuminated object at a distance of 50 cm from the screen and set up the apparatus as shown in the figure above.
2. Now place the lens somewhere close to the screen and see in which position of the lens does the illuminated object comes into focus on the screen.
3. Mark this position as L1 and record its distance from the illuminated object.
4. Now place the lens somewhere close to the illuminated object where the object comes into focus on the screen.
5. Mark this position as L2 and record its distance from the illuminated object.

Conclusion

Object and image distances should be measured along a line parallel to the principal axis.The illuminated object should be placed in the region of the optical centre.Check the focal length of the lens using the distant object method to ensure that the object distances used would be suitable.

Evaluation

The focal length of the lens is found out to be 10 cm which is very accurate to the actual value. Therefore the hypothesis stated in the beginning is proven to be correct and the focal length of the lens is found out. The actual value told is 10.4 cm so the absolute error and percentage error can be found out: -

Absolute error:

Absolute error = actual result – observed result

= 10.4 – 10.0

= 0.4

Relative error:

Relative error =

=

= 0.0385

Percentage error:

Percentage error = relative error × 100

= 0.0385 × 100

= 3.85 %

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