Difficulty: Medium

Average Score: 52%

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.

What is the main idea of the passage?

Rationale

The Doppler Effect explains why sound is heard more strongly initially and then faintly after a moving object has passed.

The passage describes how the Doppler Effect causes changes in the frequency and pitch of sound waves as a moving object, like a train or ambulance, approaches and then moves away from an observer. This scientific phenomenon is the central focus of the passage, explaining the auditory experience of hearing a sound more intensely before the object arrives and fainter as it departs.

A (Trains and ambulances emit loud noises.) YOUR ANSWER

While the passage mentions trains and ambulances, it does so to illustrate the Doppler Effect, not to emphasize the loudness of the noises themselves. The main point is the change in sound frequency and pitch due to motion, not the volume of the sounds emitted.

B (Low-frequency waves produce high-pitched sounds.) YOUR ANSWER

This statement inaccurately describes the relationship between frequency and pitch. The passage explains that low-frequency waves result from the object moving away, which produces a lower pitch, not a higher one. Therefore, this choice contradicts the scientific explanation provided.

C (High-frequency waves result in low-pitched sounds.) YOUR ANSWER

This choice incorrectly associates high frequency with low pitch. The passage actually states that high-frequency waves, which occur when an object is moving toward the observer, are associated with a higher pitch. This option misrepresents the fundamental principle of the Doppler Effect.

Conclusion

The passage's main idea centers on explaining the Doppler Effect, illustrating how it causes changes in sound perception as objects in motion approach and then move away from an observer. While other choices either misinterpret the relationship between frequency and pitch or focus on irrelevant aspects, option D accurately captures the essence of the scientific phenomenon highlighted in the passage.

Related Questions

View all