Physics quiz question: laser diffraction of hair strands

Physics 205B Quiz 3, spring semester 2016
Cuesta College, San Luis Obispo, CA

A Physics 205B student uses a red laser (wavelength 632 nm) to separately illuminate two different strands of hair. The __________ strand of hair will produce a wider central diffraction maximum.
(A) thinner.
(B) thicker.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

From the first minima equation (which uses the angle that measures one-half of the width of the central maximum on the screen), where W is the width of the obstacle (strand of hair):

W·sinθ = (1)·λ,

such that the "spread angle" is given by:

θ = sin^–1(λ/W),

so for a given wavelength of 632 nm, the thinner hair strand (smaller width W) would result in a larger "spread angle" θ.

Sections 30822, 30883
Exam code: quiz03Ccf7
(A) : 29 students
(B) : 11 students
(C) : 0 students
(D) : 0 students

Success level: 73%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50