Physics quiz question: unpolarized light through two polarizers

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

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


Unpolarized light is incident on two ideal polarizers with polarization axes turned 20° to one another. What is the fraction of the original unpolarized light intensity transmitted through the second polarizer?
(A) 0.22.
(B) 0.25.
(C) 0.44.
(D) 0.47.

Correct answer: (C)

Half of the original unpolarized light passes through the first polarizer. Of that, the fraction of light (now polarized vertically) that passes through the second polarizer with a polarization axis that is is tilted 20° with respect to this vertically polarized light is (cos(20°))² = 0.88, such that the fraction of the original unpolarized light that passes through both the first and second polarizers is 0.5·(cos(20°))² = 0.44.

Response (A) is (0.5·cos(20°))^2; response (B) is 0.5², and response (D) is 0.5·cos(20°).

Student responses
Section 70856
(A) : 1 student
(B) : 1 student
(C) : 11 students
(D) : 0 students

Success level: 85%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.25