20120214

Physics quiz question: time for light to travel a centimeter

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

How long does it take light to travel 1 inch (2.54 cm) in vacuum?
(A) 8.47×10–11 s.
(B) 1.18×10–10 s.
(C) 1.31×10–7 s.
(D) 7.62×10–6 s.

Correct answer: (C)

The speed of light in vacuum is c = 3.00×108 m/s, and the relation between speed, distance, and elapsed time is:

c = ∆x/∆t,

such that solving for time gives:

t = (∆x)/c = (3.00×108 m/s)/(1×10-2 m) = 8.47×10–11 s.

Response (B) is c/∆x (with a sign error in the exponent); response (C) is 1/(c*∆x); and response (D) is c*∆x (with a sign error in the exponent).

Section 30882
Exam code: quiz01pL2r
(A) : 17 students
(B) : 10 students
(C) : 1 student
(D) : 1 student

Success level: 59%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.83

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