A person standing on a railway platform notices that the sound of an approaching train's whistle changes in pitch (frequency) as the train passes by. This phenomenon is best explained by:

Correct — C. The Doppler effect is the change in the frequency a listener receives when the source and the listener move relative to each other.

As the train approaches, each wave crest is sent from a little closer than the last, so crests arrive more often and the pitch sounds higher. Once the train passes, they arrive less often and the pitch drops. That is option (c).

Why the others are wrong

• (a)
  Refraction of sound — Refraction bends sound where its speed changes, for example between air layers at different temperatures. It changes direction and wavelength, not the frequency a listener receives.
• (b)
  Diffraction of sound — Diffraction is sound bending around obstacles and through openings, which is why you can hear someone round a corner. It changes where sound reaches, not its pitch.
• (d)
  Resonance — Resonance is a body vibrating strongly when driven at its own natural frequency. It builds up amplitude at one frequency and does not make a pitch slide as a train passes.

Frequency is set by the source. A whistle vibrating at a fixed rate sounds the same to a listener when neither of them moves.

Motion changes what arrives. For a source moving at speed vₛ through air in which sound travels at v:
f′ = f × v ⁄ (v − vₛ) while it approaches
f′ = f × v ⁄ (v + vₛ) while it recedes

So the pitch is higher on approach, falls as the train passes and stays lower as it moves away.

Key facts

• Christian Doppler proposed the effect in 1842.
• A source moving towards a listener is heard at a higher frequency, and one moving away at a lower frequency.
• The frequency of sound does not change when it passes into another medium, but its speed and wavelength do.
• Radar speed guns, Doppler ultrasound of blood flow and astronomical redshift all use the Doppler effect.

Common traps

• Choosing refraction because the sound seems to change on its way, when refraction bends sound without changing its frequency.
• Thinking the whistle itself changes pitch, when the source frequency is constant and only the received frequency shifts.

No directly related past PYQ was found.