EXCERPTED FROM THOMASNET:  Radars are frequently used to identify distance and speed, such as how far away an object is or how fast it is moving. A radar device emits a radio wave, which moves at the speed of light, and bounces back to the radar device when it encounters an object in its path. Based on how long it takes for the radio wave to hit the designated object (for example, a tree at an undetermined distance) and how long it takes the wave to bounce back, the radar device can figure out the distance between the device and the tree. 

When a radar is used to detect speed (for example, the rate at which a car is moving), the radio wave frequency of the returned signal is altered because the car is moving. If the car is moving toward the radar device, the return signal has a shorter distance to travel and the radio wave frequency increases. The radar device can then use the change in frequency to determine the speed at which the car is moving. In laser-speed guns, waves of light are used in place of radio waves.