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A vehicle, with a horn of frequency n is moving with a velocity of 30 m/s in a direction perpendicular to the straight line joining the observer and the vehicle. The observer perceives the sound to have a frequency n + n₁. Then (if the sound velocity in air is 300 m/s)

1. n₁ = 10n
2. n₁ = 0
3. n₁ = 0.1n
4. n₁ = -0.1n

Correct answer: n₁ = 0

Solution

Since the vehicle is moving perpendicular to the line joining the observer and the vehicle, there is no relative motion along the line of sight. Hence, there is no Doppler effect, and the frequency perceived by the observer remains unchanged, making n₁ = 0.

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