Table of Contents
The Doppler Effect
The Doppler effect is the change in frequency or wavelength of a wave as perceived by an observer moving relative to the source. Named after Austrian physicist Christian Doppler who proposed it in 1842, this phenomenon is observed with all types of waves including sound, light, and water waves. The classic example is the changing pitch of an ambulance siren as it approaches and then recedes from a listener.
When a source moves toward an observer, the waves are compressed (higher frequency, shorter wavelength). When moving away, the waves are stretched (lower frequency, longer wavelength). The magnitude of the shift depends on the ratio of relative velocity to wave propagation speed. For sound waves in air, speeds are slow enough that the effect is easily noticeable. For light, very high velocities are needed to produce observable shifts.
Formulas
Where c is the wave speed (343 m/s for sound in air), v_source and v_observer are velocities (positive = toward each other), and β = v/c for electromagnetic waves.
Applications
- Radar speed guns: Measure vehicle speed from reflected wave frequency shift
- Medical ultrasound: Doppler imaging measures blood flow velocity
- Astronomy: Redshift and blueshift reveal stellar and galactic motion
- Weather radar: Doppler radar detects storm rotation and wind speeds
- Satellite tracking: Frequency changes reveal orbital parameters
Common Examples
| Scenario | Source Freq | Velocity | Observed Freq |
|---|---|---|---|
| Ambulance approaching | 700 Hz | 30 m/s toward | 767 Hz |
| Ambulance receding | 700 Hz | 30 m/s away | 644 Hz |
| Train horn | 500 Hz | 50 m/s toward | 585 Hz |
FAQ
What happens when the source moves at the speed of sound?
When the source velocity equals the speed of sound, a sonic boom occurs. The wavefronts pile up into a single shock wave, creating an intense pressure wave. The Doppler formula becomes singular (division by zero) because the denominator (c - vs) equals zero, reflecting the physical reality that all waves arrive simultaneously, creating infinite compression.
How is Doppler used to measure blood flow?
Medical Doppler ultrasound sends ultrasonic waves into the body and analyzes the frequency shift of waves reflected from moving red blood cells. The shift is proportional to blood flow velocity, allowing doctors to assess cardiac function, detect arterial blockages, and monitor fetal blood circulation. Color Doppler imaging maps velocity data onto anatomical images.
What is cosmological redshift?
Cosmological redshift is the stretching of light wavelengths from distant galaxies caused by the expansion of space itself, not by the galaxy's motion through space. Discovered by Edwin Hubble, it shows that more distant galaxies have greater redshifts, demonstrating that the universe is expanding. This is related to but distinct from the classical Doppler effect.