Ever heard the high pitch of an ambulance siren, and notice that its pitch drops suddenly as the ambulance passes you? That's the Doppler effect. Here it is in action using a trombone and skis. The change in frequency or wavelength of a wave for an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.
Years 1917-2015.
