A radar gun is a portable device used to measure the speed of something, especially a baseball or moving vehicle. Especially, radar guns are used by police to catch speeders and baseball coaches to measure the velocity of a pitcher's throw. A typical radar gun can be bought online or in a sports supply store for about $100 USD (US Dollars). Radar guns were invented in 1954 by Bryce Brown, and first used by patrolman Leonard Baldy in Chicago.
A radar gun works by bouncing radar off the object to be measured and using the Doppler effect to infer its speed. As such, they are also referred to as Doppler radars. You might be familiar with the Doppler effect by listening to a passing train or car -- especially with a train whistle, it sounds more high-pitched as the train is approaching, and becomes low pitch as the train is departing, despite the fact that the pitch of the whistle is objectively constant.
The change in pitch is caused by the motion of the object emitting the noise. When the object is moving quickly towards the listener, the sound waves are compressed by the object's increasing closeness as it emits the sound. When the object is departing, the waves are stretched out by the object's increasing distance as it emits the sound. The same principle applies to light, other forms of electromagnetic energy (such as radar waves), and any emitted wave.
Functioning both as a radar emitter and transceiver, a radar gun both emits radar waves and analyzes them upon reception. This allows very precise measurements of velocity, but a radar gun can occasionally be in error when it picks up the motion of an object in the foreground, such as a moving bird, rather than the intended target object. As a result of the risk of error, police in many states are required to complete a course on the accurate use of radar guns and receive a certification.