A simple magnifier of refractive power 15.0 D is used to observe a beetle on a student's desk. Assume that the magnifier is held close to the student's eye. If the image of the beetle is twice as large as the beetle itself, how far away is the image formed relative to the lens?
A) 6.7 cm
B) 8.3 cm
C) 3.3 cm
D) 10 cm
E) 20 cm

Question

Quizplus · Accepted Answer

The magnification (M) is given by M = 1 + (D/f), where D is the distance of the near point (usually 25 cm) and f is the focal length. Given M = 2 and D = 25 cm, solving for f gives f = 25 cm. The refractive power (P) is 15 D, so f = 1/P = 1/15 m = 6.67 cm. The image distance (v) is given by 1/f = 1/u + 1/v, where u is the object distance. Solving for v when M = 2 gives v = 6.7 cm.

A Simple Magnifier of Refractive Power 15