As seen before, a normal radar set sends out a radio pulse and waits for the reflection. Then it measures
the Doppler shift, in the signal and uses the shift to determine the speed. In the case of sound, we said
that, Doppler shift occurs when sound is generated by, or reflected off, a moving object. Doppler shift in
the extreme creates sonic booms. The Doppler shift, let’s say there is a car coming toward you at 60 kph
and its horn is blaring. You will hear the horn playing one “note” as the car approaches, but when the car
passes you the sound of the horn will suddenly shift to a lower note. It’s the same horn making the same
sound the whole time. The change you hear is caused by Doppler shift. In this case, we shall substitute
the sound with light or a laser beam; they tend to work in more or less the same way. The only difference
is that, light is much faster than sound.
Here, the Laser (or lidar, for light detection and ranging) speed guns use a more direct method that relies
on the reflection time of light rather than Doppler shift. You have probably experienced the reflection
time of sound waves in the form of an echo. For example, if you shout down a well or across a valley, the
sound takes a noticeable amount of time to reach the other side of the valley and travel back to your ear.
Sound travels at something like 300 meters per second, so a wide valley creates a very apparent round trip
time for the sound.
A laser speed gun measures the round-trip time for light to reach a car and reflect back. Light from a
laser speed gun moves a lot faster than sound at about 300,000,000 meters, or roughly 30 cm per nanosecond.
A laser speed gun shoots a very short burst of infrared laser light and then waits for it to reflect off
the vehicle. The gun counts the number of nanoseconds it takes for the round trip, and by dividing by 2 it
can calculate the distance to the car. If the gun takes 1,000 samples per second, it can compare the change
in distance between samples and calculate the speed of the car. By taking several hundred samples over the
course of a third of a second or so, the accuracy can be very high.
The advantage of a laser speed gun (for the police anyway) is that the size of the “cone” of light that the
gun emits is very small, even at a range like 1,000 feet (300 meters). The cone at this distance might be 3
feet (1 meter) in diameter. This allows the gun to target a specific vehicle. A laser speed gun is also
very accurate. The disadvantage is that the officer has to aim a laser speed gun -- normal police radar
with a broad radar beam can detect Doppler shift without aiming. If you thought that, the Policeman booking
you for overspending was a magician, now you know how he gets those speeds, no magic, is there any?