Measurement of the focal length of a converging lens

Apparatus

Converging lens, screen, lamp-box with crosswire, metre stick, retort stand.

Procedure

• Place the lamp-box well outside the approximate focal length – see notes.
• Move the screen until a clear inverted image of the crosswire is obtained.
• Measure the distance u from the crosswire to the lens, using the metre stick.
• Measure the distance v from the screen to the lens.
• Calculate the focal length of the mirror using $\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$.
• Repeat this procedure for different values of u.
• Calculate f each time and then find the average value.

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Measurement of the focal length of a converging lens

Physics Experiments

Results

$u$/cm	$\frac{1}{u}$/cm$^{-1}$	$v$/cm	$\frac{1}{v}$/cm$^{-1}$	$\frac{1}{f}$/cm$^{-1}$	$f$/cm

Average $f$ =

Note