While driving on the expressway I have passed by many police officers and like everyone around me, slowed down the second I realized it. The radar guns police use, use physics to help find out if the driver is going too fast. As the police officer aims the radar gun at cars passing by, the gun sends out radio waves toward the car. Then, the radio waves hit the car and bounce back toward the gun. The gun then measures the frequency of the returning waves, so the faster you are going toward the police radar gun, the higher frequency the waves will be. This concept uses a lot of physics including radio waves, frequency and also the Doppler effect. Since the car is moving toward the gun, the frequency of the returning radio waves will be much higher.