Physics quiz question: incident angle less than critical angle

Physics 205B Quiz 1, spring semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 23.11

A beam of light strikes the interface between flint glass and air with an incident angle equal to the critical angle of 35.9°, such that the beam undergoes total internal reflection. (Drawing is not to scale.) If the incident angle of the beam of light in flint glass is decreased slightly from 35.9° to 35.8°, the incident beam would be:
(A) reflected back into flint glass.
(B) transmitted into air.
(C) (Both choices (A) and (B).)
(D) (Not enough information is given.)

Correct answer: (C)

If the incident angle in flint glass is less than the critical angle, then light will be transmitted out into the air, as well as partially reflected back into the flint glass.

Section 30882
Exam code: quiz01pL2r
(A) : 9 students
(B) : 3 students
(C) : 17 students
(D) : 0 students

Success level: 59%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67