A person on a railway platform observes that the pitch (frequency) of an approaching train’s whistle changes as the train moves past them. Which phenomenon explains this change?
Show answer & explanation
Correct answer: C
Explanation
The correct answer is option 3 i.e. Doppler effect.
The Doppler effect is the apparent change in the frequency of a wave in relation to an observer who is moving relative to the wave source.
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As the train approaches: The sound waves are compressed, leading to a higher observed frequency (higher pitch).
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As the train moves away: The sound waves are stretched out, leading to a lower observed frequency (lower pitch).
Other Options Explained
- Refraction of sound: This is the bending of sound waves as they pass through different media or layers with varying temperature or wind speed (changing the speed of sound), which doesn't explain the sudden pitch change as the source passes.
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Diffraction of sound: This is the bending or spreading of sound waves around obstacles or through openings, which allows you to hear sound around a corner. It does not cause a change in the original frequency.
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Resonance: This occurs when a system (like a wine glass) is driven by an external frequency that matches its natural frequency, causing a large increase in amplitude (loudness), not an apparent change in the source frequency itself.