What is the Doppler effect?

The Doppler effect is defined as the change in frequency of a wave resulting from the relative motion between the source of the wave and the observer. When the source of a wave moves toward an observer, the waves are compressed, leading to an increase in frequency or pitch. Conversely, when the source moves away from the observer, the waves are stretched, resulting in a decrease in frequency or pitch. This phenomenon is commonly experienced with sound waves, such as hearing a passing siren whose pitch suddenly changes as the vehicle approaches and then recedes.

Understanding this effect is important across various fields such as astronomy, where it helps in determining the movement of stars and galaxies based on shifts in the light frequency they emit, or in radar and sonar technologies to measure speeds and distances.