The world of today is a world in motion. People constantly want to get somewhere. The heavens are filling up with satellites, our skies with airplanes and rockets, our seas with ships and submarines, and our land with cars, bikes, and trains. Humans have developed laws, rules, technologies, and subsequently entire industries to make sure all these vehicles move smoothly, without harming humans or each other. A small but significant piece of this picture is the speed gun.
What is a speed gun?
A speed gun is a device to measure the speed of a moving object without having to be in contact with the object. To achieve this, the device bounces electromagnetic radiation of a specific frequency off the object, capturing the reflection and using the Doppler effect to infer the object’s speed. Speed guns are electronic, and use complex circuitry to emit the radiation used to make the measurement.
These devices are widely used by law enforcement officials to monitor traffic speed, by coaches to gauge the performance of their athletes, and in various other industries in need of accurate motion tracking.
What is the Doppler effect?
The Doppler effect is named for the Austrian physicist Christian Doppler and relies on the simple concept of relative velocity. Say there’s a man sitting at the centre of a field blowing a whistle. The sound waves move out in a circular, concentric pattern with the whistle at the centre, and evenly spread out. A woman standing at the edge of the field will receive these waves at frequent intervals — as and when the waves’ crests reach her. (Since sound waves move at 343 m/s in air, human ears can’t hear the gaps.)
Each wave has a frequency and a wavelength. A higher frequency produces a higher pitch and vice versa.
Now, say the whistling man is moving around the field on a buggy. If the buggy is moving towards the woman, the waves in front of the vehicle become bunched up. In other words, from the woman’s perspective, the waves would have acquired both the speed of the buggy in addition to the speed of the sound wave. Thus the waves will reach the woman more frequently, and she will perceive a higher pitch. (For the same reason, the sound will have a lower pitch in a direction behind the buggy.)
This is why, when a train moves into a station, people on the platform will hear the horn blowing at a higher pitch than when the train is leaving the station. This effect is the Doppler effect.
The speed gun was originally developed during World War II for military use and applies the effect using radio waves rather than sound waves. A speed gun has a radio transmitter and a receiver. The transmitter emits radio waves, which the person holding the speed gun can direct at an object. The receiver collects the waves reflected by the object back in the direction of the speed gun.
If the object is approaching the speed gun, the frequency of the returning waves will be slightly higher than that of the transmitted waves. A simple computer in the gun can deduce the object’s speed based on this difference.
How are the speed and the effect linked?
All electromagnetic waves have a fixed speed — equal to the speed of light in that medium. In vacuum, this value is denoted c: 299,792,458 m/s. Any change in the frequency the speed gun detects directly corresponds to the Doppler shift caused by the object’s motion. This principle is powerful because it allows the speed gun to work accurately over a wide range of distances and velocities without being affected by air resistance.
A speed gun can calculate the speed of a moving object by multiplying the difference (between received and emitted frequencies) with c and dividing by the emitted frequency times 2.
This relationship shows how the difference is directly proportional to the speed of the object: the faster it moves, the more pronounced the difference will be. In other words, the only condition is that the object should be moving much slower than the speed of light — which is the case in most, if not all, practical applications of the speed gun.
Do speed guns have shortcomings?
The technology to emit radio waves is ubiquitous today. The principle is simple: when an antenna is excited by an alternating current with a radio-wave frequency, it emits radio waves. Radio-wave frequency is in the range of 30 Hz to 300 billion Hz.
For a long time, the equipment to produce the waves was bulky. This changed when scientists invented transistors in the 1940s. Electronic circuits built using transistors considerably simplified the process of producing radio waves and also made the transmitters much smaller.
However, radio waves have intrinsic shortcomings that transmitters can’t fully adjust for. For example, radio waves diverge as they move through the air. If an antenna is 5 cm long, the waves it emits will diverge by 22º to either side, producing a beam that is 44º wide overall.
Such a beam could strike more than one moving vehicle and produce inaccurate speed readings.
A continuous-wave radar — which emits radio waves and tracks their reflections continuously — may also produce readings due to multiple vehicles.
Engineers have developed systems to compensate for these errors but the resulting setups have been more sophisticated and more expensive.
For such reasons, LIDAR speed guns have been replacing radar counterparts. The name is short for ‘light detection and ranging’. LIDAR uses laser light instead of radio waves; the gun’s operation is otherwise similar. Laser light has very low divergence and thus offers better targeting.
Amartya Srinivasan is a Class XI student at P.S. Senior Secondary School, Mylapore, Chennai. Vasudevan Mukunth is deputy science editor, The Hindu.
Published – December 23, 2024 08:30 am IST
