Share:
How It Works
The Doppler effect describes how the observed frequency of a wave changes when the source or receiver is moving. When they approach each other the observed frequency rises; when they move apart it falls. The equations differ depending on whether the source, the receiver, or both are in motion relative to the medium.
This effect applies to sound waves, radar, and light. For sound in air at sea level, the wave speed is about 343 m/s.
Example Problem
An ambulance siren emits sound at 700 Hz and approaches you at 30 m/s. The speed of sound is 343 m/s. What frequency do you hear?
- λ = (343 − 30) / 700 = 0.447 m
- f′ = 343 / 0.447 ≈ 767 Hz
You hear a noticeably higher pitch as the ambulance approaches.
Frequently Asked Questions
Why does a siren sound higher when approaching?
As the source moves toward you, each successive wave crest is emitted from a closer position, compressing the wavelengths. Shorter wavelengths mean higher frequency (higher pitch).
Does the Doppler effect work for light?
Yes. In astronomy, light from stars moving away appears red-shifted (lower frequency), and light from approaching stars appears blue-shifted. This is how astronomers measure the expansion of the universe.
What is the Doppler effect used for in medicine?
Doppler ultrasound measures blood flow velocity by detecting the frequency shift of ultrasound waves reflected off moving red blood cells. This helps diagnose vascular conditions and monitor fetal heart rate.
Related Calculators
- Wien's Law Calculator — relate electromagnetic wavelength and temperature.
- Constant Acceleration Calculator — find the velocity of a moving source or receiver.
- Projectile Motion Calculator — analyze the trajectory of a moving object.
- Sound Wave Calculator — calculate wavelength, frequency, and speed of sound waves.
- Speed Converter — convert between m/s, ft/s, km/h, and other velocity units.
Reference: Tipler, Paul A. 1995. Physics For Scientists and Engineers. Worth Publishers. 3rd ed.