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CBSE Class 11-science Answered

Obtain an expression for the apparent frequency of a note heard by an observer when, (a)  source alone is in motion towards the observer. (b)  source alone is in motion away from the observer. (c)  observer alone is in motion towards the observer. (d)  observer alone is in motion away from the observer. (e)  when the source and observer are moving towards each other.
Asked by Topperlearning User | 04 Jun, 2014, 01:23: PM
answered-by-expert Expert Answer

(a)  When source alone is in motion towards an observer:

The source (S) of sound is moving towards the stationary observer (O) with velocity, say us.  Let n be the frequency of the sound waves.  Thus, the source emits n waves per second.  Let v be the velocity of the sound waves.  If the source were at rest, n waves emitted by S would have occupied a distance v (figure) in one second.  The first wave travels a distance v when nth wave is just emitted by S, as shown in the figure.  True wavelength,

As the source has moved a distance us towards O, in one second n waves emitted by S will occupy a distance (v - us) as shown in the figure (ii).  Therefore, the wavelength will be reduced.  Let the reduced wavelength be  and apparent frequency n'.

Therefore,

Answered by | 04 Jun, 2014, 03:23: PM

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