By this method, focal length of convex lens is determined. Consider an object and a screen placed at a distance D (> 4f) apart (Fig. 23). Let a lens of focal length f be placed between the object and the screen.
- For two different positions of lens two images (I1 and I2) of an object are formed at the screen.
- Focal length of the lens, where , and m1m2 = 1.
- Size of object