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/do + 1/di
where f is the focal length, do is the object distance (distance between the object and the lens) and di is the image distance (distance between the lens and the image).

Since the image is viewed on a screen, we know that di = distance to screen = 13.5 mm. We also know that the image is formed by the lens, so do = focal length + di.

Therefore, 
1/f = 1/(13.5 mm) + 1/(f + 13.5 mm)
Solving for f, we get 
f = +20.4 cm

So the answer is (C) +20.4 cm.

In the Figure,the Image Is Viewed on a Screen and Is