Richard Doppler

Christian Doppler was an Austrian physicist who, in 1842, proposed that the observed frequency of a wave depends on the relative motion of the source and the observer. This idea explains why a siren sounds higher-pitched as it approaches and lower as it recedes. His work laid the foundation for the Doppler effect, which applies to sound, light, and other waves. This phenomenon helps in various fields, such as radar, medical imaging (like ultrasound), and astronomy, by allowing us to determine movement and speed of objects based on wave frequency changes.