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 option is C

The correct answer is (c) Doppler effect.
• The Doppler effect explains the change in frequency due to relative motion.
• Frequency appears higher when the train approaches and lower when it moves away.
• It applies to sound and electromagnetic waves.
Information Booster:
• Doppler effect was proposed by Christian Doppler (1842).
• Used in radar, astronomy, and medical imaging.
• Observed only when there is relative velocity between source and observer.
Additional Knowledge:
• Refraction involves change in medium.
• Diffraction is bending of waves around obstacles.
• Resonance occurs when natural frequency matches applied frequency.