# How the Doppler Effect Works

## Practical Applications of the Doppler Effect

In the 160 years or so since Doppler first described the wave phenomenon that would cement his place in history, several practical applications of the Doppler effect have emerged to serve society. In all of these applications, the same basic thing is happening: A stationary transmitter shoots waves at a moving object. The waves hit the object and bounce back. The transmitter (now a receiver) detects the frequency of the returned waves. Based on the amount of the Doppler shift, the speed of the object can be determined. Let's look at a few specific examples.

The handheld radar guns used by police to check for speeding vehicles rely on the Doppler effect. Here's how they work:

1. A police officer takes a position on the side of the road.
2. The officer aims his radar gun at an approaching vehicle. The gun sends out a burst of radio waves at a particular frequency.
3. The radio waves strike the vehicle and bounce back toward the radar gun.
4. The radar gun measures the frequency of the returning waves. 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.
5. The difference between the emitted frequency and the reflected frequency is used to determine the speed of the vehicle. A computer inside the gun performs the calculation instantly and displays a speed to the officer.