Q: How do laser speed guns used by police to catch speeders work?
A: Conventional radar guns measure the Doppler shift, the change in frequency of the radar wave when it bounces off a moving object. Doppler shifts are responsible, for example, for the change in tone of a train's whistle as it passes you.
Laser speed guns simply measure the amount of time it takes a brief pulse of infrared light to travel to the car and back, thereby determining its distance. A few seconds later, the gun pulses again and measures the new distance. The change in distance, divided by the time, gives the car's speed.
The level of precision is remarkable, according to physicist James A. Worthey of the National Institute of Science and Technology. If the car is 500 feet away, the round-trip time for the light pulse is about one-millionth of a second. To determine speeds accurately, the gun must measure time differences in billionths of a second.