Difficulty: Medium

Average Score: 68%

Doppler Effect
Have you ever wondered why the whistle of a traveling, distant locomotive predicts its approach several yards before anyone actually sees it? Or why an oncoming ambulance's screaming siren is heard momentarily several feet before the ambulance comes into full view, before it passes you, and why its siren is still heard faintly well after the ambulance is out of sight?
What you are witnessing is a scientific phenomenon known as the Doppler Effect. What takes place is truly remarkable. In both of these instances, when the train or ambulance moves toward the sound waves in front of it, the sound waves are pulled closer together and have a higher frequency. In either instance, the listener positioned in front of the moving object hears a higher pitch. The ambulance and locomotive are progressively moving away from the sound waves behind them, causing the waves to be farther apart and to have a lower frequency. These fast-approaching modes of transportation distance themselves past the listener, who hears a lower pitch.

Which sound waves have a higher pitch?

Rationale

Those waves that are closer together have a higher pitch.

The Doppler Effect explains that when an object emitting sound moves toward an observer, the sound waves in front of it compress, leading to a higher frequency and therefore a higher pitch. This phenomenon occurs because the sound waves are squeezed closer together as the source of the sound approaches the observer, increasing the frequency perceived by the listener.

A (Those waves that are closer together) CORRECT

When sound waves are closer together, they have a higher frequency, which results in a higher pitch. This is a direct result of the Doppler Effect, as the motion of the sound source towards the observer compresses the waves.

B (Those waves that are farther apart) YOUR ANSWER

Sound waves that are farther apart have a lower frequency, leading to a lower pitch. This occurs when the sound source is moving away from the observer, causing the waves to spread out, thereby reducing their frequency.

C (Those waves that travel a long distance) YOUR ANSWER

The distance a sound wave travels does not inherently affect its pitch. Rather, pitch is determined by the frequency of the wave, not by the distance it covers. Thus, this choice does not relate to the Doppler Effect’s impact on pitch.

D (Those waves that travel a short distance) YOUR ANSWER

Similar to waves traveling a long distance, the distance covered by sound waves does not influence their pitch. Pitch is a function of frequency, and this choice does not concern the compression or expansion of sound waves relevant to the Doppler Effect.

Conclusion

The Doppler Effect describes the change in frequency (and thus pitch) of sound waves as a sound-emitting object moves relative to an observer. Sound waves that are compressed and closer together due to the source moving toward the observer have a higher frequency, resulting in a higher pitch. In contrast, waves that spread out as the source moves away have a lower frequency and pitch. Choices C and D concerning distance are irrelevant to changes in pitch as described by the Doppler Effect.

Related Questions

View all