An image of an object is formed on a screen by a lens. Leaving the lens fixed, the object...

An image of an object is formed on a screen by a lens. Leaving the lens fixed, the object is moved to a new position and the image screen moved until it again receives a focused image. If the two object positions are S1 and S2 and if the transverse magnifications of the image are M1 and M2, respectively, show that the focal length of the lens is given by

This is Abbe’s method for finding the focal length of a lens.