A police radar gun transmits microwaves at frequency f = 10.5 GHz. The waves reflect from a car approaching the gun and return with their frequency raised by 1800 Hz. Given the approximate radar formula delta_f = 2 f v_car / c (the factor 2 is for the round trip), estimate the car's speed in km/h. (c = 3.0 x 10^8 m/s.) Solution: v_car = c delta_f / (2f) = (3.0 x 10^8)(1800) / (2 x 10.5 x 10^9) = 5.4 x 10^11 / 2.1 x 10^10 = 25.7 m/s. In km/h: 25.7 x 3.6 = 92.5 km/h - over a typical 80 km/h limit, so the driver gets a ticket.