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Waves and Sounds

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Freezing a Sine Wave with Strobe Light

WAVES-XVAJKJ

A stroboscope illuminates a continuous sine wave of wavelength $\lambda$ m traveling on a long string. When the strobe is flashing at a frequency of $F$ Hz the wave on the string no longer seems to be moving--the fixed sine shape of the wave appears stationary. What expression below correctly predicts the possible value or values of the wave speed, $v$, on the string?

A

$v = \cfrac{\lambda} {F}$ is the only value for the speed.

B

$v = F \lambda$ is the only value for the speed.

C

$v = n \lambda F$ where $n$ = any positive integer.

D

$v = n \lambda F$ where $n$ = any non-zero integer positive or negative.

E

$v = n \lambda F $ where $n$ = any odd integer.