The Doppler effect

Learning objectives

Explain the Doppler effect qualitatively as the change in observed frequency and wavelength produced by relative motion between a source and an observer, using a wavefront diagram

(HL) Apply f′ = f v/(v ± us) for a moving source of sound and f′ = f (v ± uo)/v for a moving observer, selecting the correct sign for approach and for recession

Apply the approximation Δf/f ≈ Δλ/λ ≈ v/c to electromagnetic waves when the relative speed is much smaller than c, and use it to calculate a radial speed from a measured shift

Explain the red shift observed in the light from distant galaxies as evidence that the universe is expanding

Describe applications of the Doppler effect, including the measurement of vehicle speed by radar and of blood flow speed by ultrasound