Physics quiz question: unpolarized light transmitted through two polarizers

Physics 205B Quiz 1, spring semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 22.45

Unpolarized light passes through two polarizers with polarization axes turned 40° to one another.


The fraction of the incident light intensity transmitted is:
(A) 0.21.
(B) 0.29.
(C) 0.41.
(D) 0.59

Correct answer (highlight to unhide): (B)

The intensity of light that passes through polarizer 1 is one-half of the original unpolarized intensity, and this light is then vertically polarized (the convention followed in this class is to depict transmission axes of polarizers).

The fraction of the vertically polarized light that passes through polarizer 2 is:

cos²(40°) = 0.58682408883...,

thus the light that passes through both polarizer 1 and polarizer 2 is:

(1/2)·(0.58682408883...) = 0.29341204442...,

or to two significant figures, 0.29 of the original unpolarized intensity.

(Response (A) is (1/2)·sin²(40°), response (C) is sin²(40°), and response (D) is cos²(40°).)

Student responses
Section 31988
Exam code: quiz01sN3L
(A) : 3 students
(B) : 7 students
(C) : 1 student
(D) : 2 students

Success level: 53%
Discrimination index (Aubrecht & Aubrecht, 1983): 1.00