At a point, beats frequency of `n` Hz is observed it means:

Options

A. medium particles at the point, are vibrating with frequency `n` Hz.
B. amplitude of vibrations changes simple harmonically with frequency `n` Hz at that point only
C. at that zero intensity is observed `2n` times per second.
D. none of these

Answer

D

Explanation

Beats are observed when intensity at a point varies with time and beat frequency is equal to the frequency of oscillations of intensity at the point. Intensity at a point not only depends upon the frequency of medium particle also. Therefore, beat are observed when amplitude of oscillation of medium particles varies with time. If the beat frequency at a point is equal to `n`, it means, at that point amplitude of oscillation of medium particles varies with frequency `n`. Amplitude of vibrations changes not only at the point of observation, but at all the point. Therefore, (a) and (b) are wrong. Since the frequency of variation of intensity is observed `n` times per second therefore, the maximum of intensity is observed `n` times per second and the intensity become zero `n` times per second. Hence (c) is also wrong. Obviously, only option (d) is correct.