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Last updated date: 27th Nov 2023
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# Assertion: When a source of sound passes us, whether it be a car horn or a train whistle, the pitch we hear goes from high to low.Reason: According to Doppler's effect, there is apartment change in the frequency of sound observed due to relative motion between the observer and the source of sound.A. Both assertion and reason are correct and reason is the correct explanation of the assertionB. Both assertion and reason are correct but reason is not the correct explanation of the assertionC. Assertion is correct but reason is incorrectD. Both assertion and reason are incorrect

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Hint: To answer this question, we first need to understand what is pitch. Pitch is the position of a single sound in the entire range of sound in music. The pitch of sounds is determined by the frequency of vibration of the sound waves that produce them.

Doppler’s effect:The Doppler effect is a common occurrence that we see in our daily lives. The Doppler effect is defined as a shift in wave frequency (whether light or sound) as a result of relative motion between the wave source and the observer.The observed pitch varies from high to low when a sound source, such as a car horn or a train whistle, passes us.

The Doppler effect is to blame for this. When a source, such as a car horn or a train whistle, is approaching us, i.e., when the source is approaching the observer at rest, the observed pitch (${v_0}$) appears to be higher than the true pitch ($v$), because
${v_0} = \dfrac{v}{{v - {v_s}}}v$

The observed pitch appears to be less than the actual pitch when the source of sound has crossed us (i.e., the source is receding away from the observer). This is because the observed pitch appears to be less than the true pitch when the source of sound has crossed us (i.e., the source is receding away from the observer), because
${v_0} = \dfrac{v}{{v + {v_s}}}v$
Both assertion and reason are correct and reason is the correct explanation of the assertion

Hence, the correct answer is option A.

Note: In RADAR sensors, the Doppler’s effect is employed to quantify speed. The frequency of the reflected radio wave is modified when a fixed-frequency radio wave sent from the sender constantly contacts an object moving towards or away from the sender. The Doppler effect is the name for this frequency shift.