Question

Which of the following about Doppler Effect is/are correct?

a) You are at the center of a circle, and a speaker is moving around the circle. You hear an increase in frequency.

b) Car A and Car B travel at identical speeds, and Car A is behind Car B traveling in the same direction. If Car A activates a sound source, Car B will detect a reduced frequency because it is ahead of Car A.

c) A fire truck sets off its siren as it moves toward a mountain. The frequency of the reflected sound waves detected by the fire truck will be at a higher frequency than the original sound waves emitted.

Answer 1

C) A fire truck sets off its siren as it moves toward a mountain. The frequency of the reflected sound waves detected by the fire truck will be at a higher frequency than the original sound waves emitted.

As truck is approaching the hill hence for reflection source and observer are approaching each other. Therefore the apparent frequency is more than its original frequency.

In case of a) the velocity of sound and source is perpendicular all the time hence no change in frequency of sound. No Doppler effect.

In case b) relative velocity between source and sound is zero. Hence no change in frequency. No Doppler effect.