Doppleri

Doppleri, commonly referred to as the Doppler effect, is the change in frequency or wavelength of waves that occurs when there is relative motion between the source of the waves and an observer. Named after the Austrian physicist Christian Doppler, who proposed it in 1842, the effect applies to all waves, including sound and electromagnetic radiation. In general, observed frequency increases when the source and observer move toward each other and decreases when they move apart. The effect can also arise from motion of the medium through which the wave travels, which is relevant for sound.

For sound in a medium such as air, the observed frequency f' is approximately f' = f (v + vo) / (v − vs), where v is the wave speed in the medium, vo is the observer’s speed toward the source, and vs is the source’s speed toward the observer. Intuitively, approaching sources produce a higher pitch and receding sources a lower pitch.

For light, the Doppler effect is described by the theory of relativity. The relativistic formula relates observed and emitted frequencies as f' = f sqrt((1 + β) / (1 − β)), with β = v/c along the line of sight. A receding source yields a redshift, an approaching source a blueshift. At low velocities (v ≪ c), the relativistic result reduces to a classical approximation.

Applications of the Doppleri effect are widespread, including astronomy (measuring stellar and galactic radial velocities and redshift), radar and lidar, weather radar, and medical ultrasound for blood flow assessment. It provides a practical tool for inferring motion and velocity in diverse contexts.