Sound and Waves PYQs – Physics, SSC & Railway

The audible frequency range for a healthy human ear is generally 20 Hz to 20,000 Hz (20 kHz).

Wavelength λ = v / f.
λ = 340 / 680 = 0.5 m.

When sound passes between media, the frequency of the wave remains the same (set by the source).
Since v = f × λ, change of speed in water (which is greater than in air) changes the wavelength.
Hence frequency unchanged, wavelength varies.

When a source of sound moves relative to an observer, the observed frequency changes.
When the train approaches: frequency (pitch) appears higher.
When the train moves away: frequency appears lower.
This change in observed frequency due to relative motion between source and observer is called the Doppler Effect.
Answer: Doppler effect

Correct Answer: Option 2 – A stationary observer hears a change in the pitch of a siren as an ambulance passes by
This is the classic demonstration of the Doppler effect for sound: as an ambulance approaches, the sound waves compress (higher frequency = higher pitch); as it moves away, waves stretch (lower frequency = lower pitch). The observer hears a drop in pitch as the ambulance passes.
Option 1 (Redshift of starlight): This IS the Doppler effect, but for light (electromagnetic waves), not sound. It is specifically called 'redshift' and indicates the star is moving away from the observer.
Option 3 (Water waves bending in shallow water): This describes refraction of waves, not the Doppler effect.
Option 4 (Sunlight through prism): This describes dispersion of light (separation into spectrum by wavelength); not the Doppler effect.