A police car is traveling at a constant speed of 100. km/h with a radar gun which generates and detects radiowaves. The police car and the ambulance drive directly towards each other. The radar gun produces radiowaves with a wavelength of 0.0650 m. What doppler shift (Δf = f′–f) is detected by the radar gun after the radiowave has been reflected by the ambulance? I don't really know what to do. Do I use the relative speed between the police car and ambulance which is 180kmph? The right answer is 1540 Hz.