A) \[\frac{2u}{\lambda }\]
B) \[\frac{u}{\lambda }\]
C) \[\frac{u}{2\lambda }\]
D) \[\frac{\lambda }{u}\]
Correct Answer: A
Solution :
Let \[v\] be the speed of sound and \[n\] the original frequency of each source. They emit sounds of wavelength \[\lambda \] When observer moves towards one source (say A), the apparent frequency of A as observed by the observer will be \[n'=n\left( \frac{v+u}{v} \right)\] The observes is now receding source B, so die apparent frequency of S observed will be \[n'=n\,\left( \frac{v-u}{v} \right)\] Thus, number of beats \[x=n'-n''\] \[=n\left[ \frac{v+u}{v}-\frac{v-u}{v} \right]\] \[=\frac{n}{v}[v+u-v+u]\] \[=\frac{2nu}{v}\] but \[v=n\lambda \] \[Thus,x=\frac{2nu}{n\lambda }=\frac{2u}{\lambda }\]You need to login to perform this action.
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