Question:
The apparent frequency of a siren increases as the source approaches a stationary observer. This phenomenon is known as
The apparent frequency of a siren increases as the source approaches a stationary observer. This phenomenon is known as
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Remember that for an approaching source, the observed frequency increases due to the compression of wavefronts. For a receding source, the observed frequency decreases because the wavefronts are stretched out.
Updated On: May 31, 2026
- Doppler Effect
- Resonance
- Interference
- Diffraction
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The Correct Option is A
Solution and Explanation
Step 1: Concept
The Doppler Effect refers to the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. For sound, this effect causes the observed frequency to increase when the source approaches the observer and decrease when it moves away.
Step 2: Meaning
In the context of the question, as the siren (source) approaches a stationary observer, the distance between successive wave crests decreases, leading to an increased number of waves reaching the observer per unit time. This results in a higher perceived frequency by the observer.
Step 3: Analysis
Option A: Doppler Effect - This is correct because it explains why the apparent frequency increases as the source approaches. Option B: Resonance - This occurs when the natural frequency of an object matches the frequency of an external force, which does not apply to the scenario described. Option C: Interference - This involves the combination or cancellation of two or more waves. The question describes a single source approaching an observer, so interference is not relevant here. Option D: Diffraction - This refers to the bending of waves around obstacles or through openings. It does not explain the change in frequency as the siren approaches.
Step 4: Conclusion
The phenomenon described by the increasing apparent frequency as the siren approaches a stationary observer is best explained by the Doppler Effect. Final Answer: (A)
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