An object is placed on an optical bench. A lens with a focal length of 12 cm is placed 20 cm from the object. Where should a second lens, with a focal length of 4.0 cm, be placed if a total magnification +6 is to be obtained?
A)20 cm from the first lens
B)35 cm from the first lens
C)40 cm from the first lens
D)30 cm from the first lens
E)25 cm from the first lens

Question

Quizplus · Accepted Answer

The total magnification is given by the product of the magnifications of the two lenses. The magnification of the first lens is -20/12 = -5/3, and the magnification of the second lens is 4/x, where x is the distance between the second lens and the object. The total magnification is therefore (-5/3)(4/x) = +6. Solving for x gives x = 35 cm.

An Object Is Placed on an Optical Bench