When an object is placed 60. cm from a converging lens, it forms a real image. When the object is moved to 40. cm from the lens, the image moves 10. cm farther from the lens. What is the focal length of the lens?
A) 20. cm
B) 40. cm
C) 30. cm
D) 50. cm
E) 42. cm

Question

Quizplus · Accepted Answer

The lens equation is 1/f = 1/v - 1/u, where f is the focal length, v is the image distance, and u is the object distance. When the object is at 60 cm, let the image distance be v1. When the object is at 40 cm, the image distance becomes v1 + 10. Using the lens equation for both positions and solving for f gives a focal length of 20 cm.

When an Object Is Placed 60