Question

When source of sound moves towards a stationary observer, the wavelength of sound received by him
A. decreases while frequency increases
B. remains the same whereas frequently increases
C. increases and frequency also increases
D. decreases while frequency remains the same

Answer 1

As the source approaches a stationary observer the velocity of observer will be 0 and henceforth there will be changes in wavelength and frequency. If wavelength increases the frequency decreases or vice versa as they are inversely proportional.

Complete step by step answer:
The apparent change in the frequency of wave motion due to the relative motion between the source and the observer is called as Doppler effect
We have,
${{{\upsilon }}_{\text{o}}}{\text{ = }}{{{\upsilon }}_{\text{s}}}\left( {\dfrac{{{{v \pm }}{{\text{v}}_{\text{o}}}}}{{{{v \pm }}{{\text{v}}_{\text{s}}}}}} \right)$ where ${{{\upsilon }}_{\text{o}}}{\text{, }}{{{\upsilon }}_{\text{s}}}$ is the frequency of observer and source
${{\text{v}}_{\text{o}}}{\text{,}}{{\text{v}}_{\text{s}}}$ is the velocity of observer and source
+ is used when observer is moving towards or source receding
- is used when observer is moving away or source approach
Here the observer is stationary thus ${{\text{v}}_{\text{o}}}{\text{ = 0}}$
From the above equation ${{{\upsilon }}_{\text{o}}}{\text{ = }}{{{\upsilon }}_{\text{s}}}\left( {\dfrac{{\text{v}}}{{{\text{v - }}{{\text{v}}_{\text{s}}}}}} \right)$ since the source is moving towards we use ${\text{v - }}{{\text{v}}_{\text{s}}}$ Thus the wavelength of sound received by the observer is decreasing with the increase in frequency.
Thus the correct answer is A.

Additional information:
Doppler effect in light:
The speed of light is so high that the celestial bodies moving at high velocities show the Doppler effect. The effect that is observed consists of a shift in the wavelength of light emitted by celestial bodies
Doppler effect is sound is used in the determination of velocity of submarines
It is also used in tracking of the artificial satellites

Note:
The Doppler effect is used to detect the speed of the vehicles with the help of radio waves and the instrument used for this purpose is called a radar gun. Doppler effect also finds its applications in astrophysics to study galaxies, binary stars etc.