Physics quiz question: reflection phase shifts

Physics 205B Quiz 3, Spring Semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 25.17

A thin film of oil (n = 1.50) is spread over a puddle of water (n = 1.33). Incident light of a given wavelength in air reflects with destructive interference. Light that reflects off the top of the film has a phase shift of __________, while the light that reflects off the bottom of the film (and the top of the water) has a phase shift of ___________.
(A) 0°, 0°.
(B) 0°, 180°.
(C) 180°, 0°.
(D) 180°, 180°.

Correct answer: (C)

Light in air reflects off of the top of the oil film with a 180° phase shift ("fast off of slow"). Light in oil reflects off the top of the water with no phase shift ("slow off of fast"). (The two reflections are thus out of phase with respect to each other.)

Student responses
Section 31988
(A) : 0 students
(B) : 4 students
(C) : 5 students
(D) : 2 students

Success level: 45%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67

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Online homework assignment: destructive thin film reflections (Spring semester 2010).