Physics quiz question: reading glasses used as simple magnifier

Physics 205B Quiz 2, spring semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 24.29

A Physics 205B student would like to use a lens from +5.0 D reading glasses as a simple magnifier. What is the angular magnification of this lens as used by a relaxed eye?
(A) +0.80.
(B) +1.3.
(C) +5.0.
(D) +20.

Correct answer (highlight to unhide): (B)

The angular magnification of a converging lens used as a simple magnifier (eyepiece) is given by:

M = N/f,

where N = 0.25 m is the nominal near point, and f is the focal length (in m) of the lens, which is the reciprocal of the refractive ("optical") power P (in diopters, D or m^-1). Thus:

M = N/f = N·P = (0.25 m)·(+5.0 m^-1) = +1.25,

or +1.3, to two significant figures.

Response (A) is f/N = 1/(P·N); response (C) is the refractive power P; and response (D) is P/N = 1/(f·N).

Student responses
Section 31988
(A) : 0 students
(B) : 5 students
(C) : 4 students
(D) : 3 students

Success level: 41%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.60