An object is placed 100 cm in front of a lens of focal length 20 cm. A second lens is placed 15 cm past the first lens. The second lens has a focal length of 40 cm. Where is the final image formed?
A)25 cm in front of the second lens
B)8 cm in front of the second lens
C)8 cm after the second lens
D)10 cm after the second lens
E)25 cm after the second lens

Question

Quizplus · Accepted Answer

First, use the lens formula for the first lens to find the image distance: 1/f = 1/v - 1/u, where f = 20 cm and u = -100 cm. This gives v = 25 cm. The image from the first lens acts as the object for the second lens, located 25 - 15 = 10 cm in front of it. Using the lens formula for the second lens with f = 40 cm and u = -10 cm, the final image distance v is found to be 8 cm after the second lens.

An Object Is Placed 100 Cm in Front of a Lens