Question

According to the Doppler effect, which of the following changes apparently?
A) loudness
B) pitch
C) amplitude
D) velocity
E) acceleration

Answer 1

The Doppler effect is the variation in the frequency of sound due to the source and observer's relative movement. The sensation of the sound on a surface is called the pitch of the source. The pitch varies corresponding to the frequency of the sound.

Complete step by step answer:
Let a source of the sound move towards the stationary observer as the source approaches the observer; the distance between the source and observer decreases. The sound's speed remains constant, so the source's sound reaches an observer in less time. The frequency of the sound for the observer increases due to a decrease in the distance. The pitch of the sound also increases due to an increase in the frequency.
If the source moves away from the observer, then the distance between the source and observer increases for the same sound. The sound takes more time to reach the observer, and the sound's frequency appears to decrease.
If the source and observer both are moving, then the frequency of the source also varies. The sensation of the sound for the observer also changes.

Therefore, the sound changes pitch, apparently due to variation of frequency in the Doppler effect, and the option (B) is correct.

Additional Information:
The Doppler effect also observers in the light due to variation in the light's frequency or wavelength. The redshift in the light is used to study the relative movement of the galaxies.

Note:
- Be careful in calculating the variation frequency as the frequency increases on decreasing the distance between the source and observer. We can use the following formula to calculate the variation in the frequency.
$f' = \left( {\dfrac{{v \pm {v_0}}}{{v \mp {v_s}}}} \right)f$
Here, f is the frequency of the source, $f'$ is the apparent frequency of the source, v is the speed of the sound, ${v_o}$ speed of the observer, ${v_s}$ is the speed of the source.