Physics quiz question: diamond-air interface total internal reflection

Physics 205B Quiz 1, spring semester 2017
Cuesta College, San Luis Obispo, CA

Light with an incident angle of 28.0° strikes the interface between diamond (index of refraction[*] 2.418) and air (index of refraction 1.000), and is totally internally reflected back down into diamond. (Drawing is not to scale.) As measured from the normal, the incident angle of 28.0° is __________ the critical angle for the diamond-air interface.
(A) less than.
(B) equal to.
(C) greater than.
(D) (Not enough information is given.)

[*] physics.info/refraction/.

Correct answer (highlight to unhide): (C)

Since the incident ray is totally internally reflected, the incident angle of 28.0° must be greater than the critical angle.

(The critical angle for light incident on a diamond-air interface is given by:

θ_c = sin^–1(n₂/n₂) = sin^–1(1.000/2.418) = 24.43°.)

Student responses
Sections 30882, 30883
Exam code: quiz01Om6A
(A) : 8 students
(B) : 2 students
(C) : 18 students
(D) : 0 students

Success level: 64%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.75