Question

An object is placed 20.0 cm to the left of a converging lens with focal length of 10.0cm. A second identical, lens is placed to the right of the first lens and moved until the final image produced is identical in size and orientation to the object. What is the separation between the two lenses?

Answer 1

Since the combination gives the magnification of 1 and object is at 2f distance from the first lens. The image by first lens will form at 2f behind the lens. The magnification of the first lens will be -1, i.e image is inverted and identical. Now the second lens also must have the -1 magnification to give final magnification is +1. This is only possible if the image from first lens is at 2f distance from second lens. So the total distance between lenses is 2f+2f=4f=4x10=40 cm.