Question

6. Explain how the Doppler effect is used by the police to measure the speed of a car.

Answer 1

How does this work. Actually Police uses a radar gun to detect the speed of moving vehicles which works on the principle of doppler effect. The Doppler effect describes the change in the observed frequency of a wave when there is relative motion between the wave source and the observer.

How does it work :

A police officer usually takes a position on the side of the road to have a watch on moving cars. The officer aims his radar gun at an approaching vehicle. The gun sends out a burst of radio waves at a particular frequency. The radio waves strike the vehicle and bounce back toward the radar gun. The radar gun measures the frequency of the returning waves.

Now, Because the car is moving toward the gun, the frequency of the returning waves will be higher than the frequency of the waves initially transmitted by the gun. The faster the car's speed, the higher the frequency of the returning wave. The difference between the emitted frequency and the reflected frequency is used to determine the speed of the vehicle. However, if policeman stands behind the vehicle, he will measure velcoity lower than actual velocity.