Experiment to determine the focal length of a converging lens using the lens formula

Answers

Procedure:
1. Set up the apparatus as shown below

2. Place the object at reasonable length from the screen until a real image is formed on the screen. Move the lens along the metre rule until a sharply focused image is obtained.
3. By changing the position of the object obtain several pairs of value of u and v and record your results as shown.
u--v--u v--u v / u + v

Discussion:
The value u v / u + v is the focal length of the lens and the different sets of values give the average value of ‘f’. Alternatively the value ‘f’ may be obtained by plotting a graph of 1 / v against 1 / u. When plotted the following graph is obtained.

Since 1 / f = 1 / u + 1 / v, at the y-intercept 1 / u = 0, so that 1/ f = 1 / v or f = v.
The focal length may therefore be obtained by reading off the y-intercept and finding the reciprocal. Similarly at the x-intercept, 1 / v = 0, therefore 1 / f = 1 / u or f = u hence the focal length can also be obtained by reading off the x-intercept and finding the reciprocal.

Mutiso answered the question on October 15, 2018 at 18:12

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