3. Sound and Waves

3.1 Doppler Effect

Scientist: Christian Doppler

Year / Era: 1842

Statement: The apparent frequency (pitch) of a wave changes for an observer when there is relative motion between the source of the wave and the observer — the frequency appears higher when the source and observer approach each other, and lower when they move apart.

Explanation: Although originally proposed for sound waves, the Doppler effect applies to all types of waves, including light. When a sound source moves toward a stationary observer, successive sound wave crests are emitted from positions progressively closer to the observer, compressing the wavelength and raising the perceived pitch. The opposite occurs when the source recedes.

Formula: f' = f × (v ± vo)/(v ∓ vs), where f is source frequency, v is wave speed, vo is observer velocity, and vs is source velocity.

Application/Example: The classic example is the changing pitch of a train's or ambulance's siren as it passes a stationary listener — the pitch is higher as it approaches and lower as it recedes. The effect is also used in Doppler radar for weather forecasting and speed-measuring police radar guns, and in astronomy, where the 'redshift' of light from distant galaxies (a form of Doppler effect) reveals that the universe is expanding.