Assertion In Doppler’s effect, if the source is moving, change in frequency is observed due to change in…
Assertion In Doppler’s effect, if the source is moving, change in frequency is observed due to change in wavelength.
Reason By the motion of source, wavelength always decreases.
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If Assertion is incorrect but Reason is correct.
Solution
When a source moves towards the observer the spacing between successive wavefronts in front of the source is reduced, so the effective wavelength is changed. For a source moving with speed $v_s$ in a medium where wave speed is $v$, the wavelength ahead of the source is
$\lambda'=\dfrac{v-v_s}{f}$.
(Statement explains that wavelength is modified when the source moves and gives the expression for the effective wavelength.)