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14. waves and sound — Physics STD 11 Science — Question

Rajasthan BoardEnglish MediumSTD 11 SciencePhysics14. waves and sound1 Mark
MCQ
A source of sound $S$ having frequency $f.$ Wind is blowing from source to observer $O$ with velocity $u$. If speed of sound with respect to air is $C,$ the wavelength of sound detected by $O$ is: 
  • ✓
    $\frac{{C + \,u}}{f}$
  • B
    $\frac{{C - u}}{f}$
  • C
    $\frac{{C\left( {C + u} \right)}}{{\left( {C - u} \right)f}}$
  • D
    $\frac{C}{f}$
✓

Answer

Correct option: A.
$\frac{{C + \,u}}{f}$
a
Net velocity of sound $w.r.t$ ground, $v^{\prime}=v_{s}+v_{m}$

where $v_{s}$ is the velocity of sound $w.r.t$ medium and $v_{m}$ is the velocity of the medium.

Thus $v^{\prime}=C+u$

As both the source as well as the observer are at rest, so frequency will remain the same as $f$ irrespective of the velocity of medium.

Let $\lambda$ be the wavelength of the sound wave detected by observer.

using, $v^{\prime}=f \times \lambda$

$C+u=f \lambda \Longrightarrow \lambda=\frac{C+u}{f}$

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