Most of us have probably heard of the Doppler effect or the Doppler shift, but did not look too much into it. But did you know we encounter this effect daily? Here’s how:
The Doppler effect may not be tangible, but we can observe it in a number of settings. Say, you are standing by the sidewalk and a car speedily approaches, the frequency of the sound waves you hear would grow until the car passes you, then it would abruptly stop. This is similar to how Austrian physicist Christian Doppler discovered the Doppler effect in the 19th century, albeit with the sound of a train passing through a station.
As the source of the sound moves, it is no longer at the center of the sound waves coming from it. Instead, the circle of waves move forward along with it, compressing the waves toward the front. A person standing ahead of the moving sound source hears the compressed sound waves in a much higher frequency than the waves produced by the source if it was stationary.
Meanwhile, someone standing some distance behind it hears the sound from behind the object, but at a much lower frequency, since the waves are compressed in front of the source and spreads out behind it.
Meteorologists are able to forecast the weather with the help of Doppler radars, with each of them made up of a transmitter and a receiver. The transmitter produces pulses of radio waves outward and in a circular pattern. Precipitation in the atmosphere (e.g. rain, hail, and snow) would then scatter the waves and send some of them back, to be detected by the radar’s receiver.
The radar echo, which is the intensity of the signal received by the radar, then indicates how heavy or intense the detected precipitation is, which could be an approaching storm. By measuring the time it took for the waves to leave and return to the radar, the distance of the storm can then be identified, and the direction at which the radar points would locate exactly where the storm is.
A Doppler radar can measure the wind speed in the precipitating region, as well. They could “hear” high-frequency waves if particles of precipitation are blown towards the radar, and low-frequency waves if the particles are moving away. This “hearing” allows Doppler radars to identify severe weather. For instance, particles that keep moving towards and then away from the radar at a limited distance might be coming from a tornado.
Speed guns used in law enforcement and sports apply the same principle as meteorologists’ Doppler radars. The change in frequency resulting from the Doppler effect is twice the ratio between the velocity of a moving target and the speed of the radar pulse directed at it.
When pointed at a speeding car, the speed gun used by police officers are able to calculate the car’s speed using that method. The same goes for Stalker radar guns that many professional baseball players use to measure the speed of a baseball pitch.
When Christian Doppler presented the Doppler effect in a paper he wrote in 1842, he may not have imagined that it would apply to cars, weather forecasting, law enforcement, and sports. We might find more useful applications of the Doppler effect in the future, who knows?