The Doppler effect · Grade 9
When a source of waves moves towards you, the waves get squashed together so you receive a higher frequency; when it moves away they are stretched and the frequency drops. You hear it as a siren’s pitch falls as it passes. The same effect in light – red shift and blue shift – tells astronomers how fast stars and galaxies are moving.
Key facts
- Source approaching: higher observed frequency (shorter wavelength). Receding: lower frequency.
- Source moving, listener still: f_L = v f_s ÷ (v ∓ v_s) (− when approaching, + when receding).
- Listener moving, source still: f_L = (v ± v_L) f_s ÷ v (+ when approaching).
- The speed of the sound itself does not change; only the received frequency and wavelength do.
- Light: red shift z = Δλ ÷ λ ≈ v ÷ c for speeds much less than c.
- Uses: speed cameras (radar), measuring blood flow with ultrasound, finding exoplanets and galaxy speeds.