Lenses-Experiment 1

Measure the focal length of your lens by producing an image of the distant trees on a white screen.

f

Use strongly illuminated crosshairs as your object.
•	Place it  at a distance, u,   greater than 2 focal lengths from the lens axis  and produce a sharp image of the cross hairs on the screen. 
•	Measure the image distance v from the lens axis 
•	Record your results for u and v in the table.
•	Repeat the experiment for systematically for smaller values of u until the cross hairs lie within one focal length of the lens.
•	Plot a graph of 1/u (x axis) against 1/v (y axis). 
•	Calculate the gradient of your graph. 
•	Find the intercept on the y axis.
•	Try to find a relationship between this intercept and the focal length of the lens.

cross hairs

v

u

object distance u/cm	image distance v/cm