Since with electromagnetic radiation like microwaves or with sound, frequency is inversely proportional to wavelength, the wavelength of the waves is also affected. If the pitcher moves at an angle, but at the same speed, the frequency variation at which the receiver catches balls is less, as the distance between the two changes more slowly.įrom the point of view of the pitcher, the frequency remains constant (whether he's throwing balls or transmitting microwaves). The catcher catches balls less frequently because of the pitcher's backward motion (the frequency decreases). The inverse is true if the pitcher is moving away from the catcher. However, if the pitcher is jogging towards the catcher, the catcher catches balls more frequently because the balls are less spaced out (the frequency increases). Assuming the balls travel at a constant velocity and the pitcher is stationary, the catcher catches one ball every second. Imagine a baseball pitcher throwing one ball every second to a catcher (a frequency of 1 ball per second). This variation of frequency also depends on the direction the wave source is moving with respect to the observer it is maximum when the source is moving directly toward or away from the observer and diminishes with increasing angle between the direction of motion and the direction of the waves, until when the source is moving at right angles to the observer, there is no shift. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.

It is commonly heard when a vehicle sounding a siren approaches, passes and recedes from an observer. The Doppler effect (or Doppler shift), named after Austrian physicist Christian Doppler who proposed it in 1842, is the difference between the observed frequency and the emitted frequency of a wave for an observer moving relative to the source of the waves. This is only a component of the real speed (170 km/h). The emitted signal toward the car is reflected back with a variation of frequency that depend on the speed away/toward the radar (160 km/h).

Doppler radar in motion map