Physics quiz question: time for light to travel an inch

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

Given that the speed of light in vacuum is 3.00×10⁸ m/s, how long does it take light to travel an inch (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 (highlight to unhide): (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 = (1 inch)/(3.00×10⁸ m/s) = (2.54×10^–2 m)/(3.00×10⁸ m/s) = 8.466666...×10^–11 s,

which to three significant digits is 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