Apparatus
Converging lens, screen, lamp-box with crosswire, metre stick, retort stand.
.
Procedure
1. Place the lamp-box well outside the approximate focal length – see notes.
2. Move the screen until a clear inverted image of the crosswire is obtained.
3. Measure the distance u from the crosswire to the lens, using the metre stick.
4. Measure the distance v from the screen to the lens.
5. Calculate the focal length of the lens using vuf 111 =+ .
6. Repeat this procedure for different values of u.
7. Calculate f each time and then find the average value.
Screen
u v
Lamp-box with crosswire Lens
Results
Average f =
Notes
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 that can be used in daylight. Cover the glass of the lamp with a piece of tracing paper. The tracing paper can be attached with some bluetack. Use ‘peel-and-stick’ letters to create an ‘object’ on the tracing paper. If the ‘object’ is a simple three-letter word then the inversion of the image will be obvious.
u/cm
u 1
/cm-1 v/cm
v 1
/cm-1
f 1
/cm-1 f/cm
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