How to Calculate Doppler Effect
What is Doppler Effect?
The Doppler effect is the change in frequency (pitch) of a wave when the source or observer is moving relative to the medium. A sound source moving toward you has a higher apparent frequency; moving away has lower frequency. Used in radar, medical ultrasound, and astronomy.
Step-by-Step Guide
- 1Observed frequency: f_obs = f_source × (v + v_observer) / (v + v_source)
- 2v = speed of sound (~343 m/s in air at 20°C)
- 3v_source positive = moving away from observer; negative = moving toward
- 4Redshift in astronomy: distant galaxies' light shifts to lower frequencies as they recede
Worked Examples
Input
Ambulance siren 700Hz · speed 30 m/s approaching
Result
Observed frequency ≈ 765Hz
700 × 343/(343−30) = 700 × 1.096 = 767
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