Question

An object is placed 60 cm in front of a convex lens with a focal length of 20 cm. Where does the image form?

A. 15 cm
B. 30 cm
C. -15 cm
D. -30 cm

Answer 1

We are given with the focal length f equal to 20 cm and distance of the object do equal to 60 centimeters. We can compute for the distance of the image di from the lens equation
1/f = 1/do + 1/di
1/di = 1/f - 1/do = (do - f) / do f
di = do f / (do - f)
Substituting the values to the derived equation, we can now calculate for the image distance di:
di = do f / (do - f)
di = 60*20 / (60 - 20) = 30 cm
Therefore, the convex lens will produce an image at 30 centimeters.