An object is placed 20 cm in front of a converging lens of focal length 10 cm. Find the image distance.
For a lens the relation between object distance, image distance and the focal length is given by 1/Do + 1/Di = 1/f, where Do is the object distance, Di is the image distance and f is the focal length. For a converging lens the focal length is taken to be positive.
As the focal length of the converging lens is 10 cm and the the object is placed at a distance of 20 cm in front of the lens, Do = 20 and f = 10. Using the relation provided earlier:
1/20 + 1/Di = 1/10
=> 1/Di = 1/10 - 1/20 = 1/20
=> Di = 20 cm
The image is formed at a distance of 20 cm from the lens on the side opposite to that where the object is placed.