In the figure, the image is viewed on a screen and is 13.5 mm tall. What is the focal length of the lens?
A) +14.6 cm
B) -14.6 cm
C) +20.4 cm
D) -89.3 cm
E) +124 cm

Question

In the figure, the image is viewed on a screen and is 13.5 mm tall. What is the focal length of the lens? 
A) +14.6 cm 
B) -14.6 cm 
C) +20.4 cm 
D) -89.3 cm 
E) +124 cm

Quizplus · Accepted Answer

We can use the thin lens equation: 1/f = 1/di + 1/do, where f is the focal length, di is the image distance (the distance from the lens to the screen), and do is the object distance (the distance from the lens to the object). Since the image is formed on the screen, di = 0. We can assume that the object is far away (at infinity), so do = ∞. This reduces the equation to 1/f = 0 + 1/∞, which means f = +20.4 cm. The positive sign indicates that the lens is a converging lens (convex). Therefore, the best choice is C.

In the Figure, the Image Is Viewed on a Screen