Last Updated Sep 27, 2025

Sample Answer for Measurement of the focal length of a concave mirror

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Measurement of the focal length of a concave mirror

Apparatus

Concave mirror, screen, lamp-box with crosswire

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 mirror, using the metre stick.
• Measure the distance v from the screen to the mirror.
• 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 an average value.

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Measurement of the focal length of a concave mirror

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

infoNote

The approximate method for finding the focal length is recommended as a starting point for this experiment. The approximate method is described in the Appendix.

A microscope lamp makes a very suitable strong light source. Cover the glass of the lamp with a piece of tracing paper. Use 'peel-and-stick' letters to create an 'object' on the tracing paper.

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