Focal length of a convex lens

A/L Physics · G.C.E. Advanced Level · Grades 12–13

Aim

To find the focal length of a convex lens by the u–v method.

Apparatus

• A convex lens
• Illuminated object and a screen
• Optical bench or metre rule

Method

1. Place the object at a distance u from the lens.
2. Move the screen until a sharp image forms.
3. Measure the image distance v.
4. Repeat for 5 values of u and calculate 1/u and 1/v.
5. Plot 1/v against 1/u; the intercept = 1/f.

Conclusion

1/u + 1/v = 1/f. The graph is a straight line with gradient −1 and intercept 1/f, which gives f.