Dopplers

Dopplers refer to the Doppler effects, the changes in frequency or wavelength of waves caused by the relative motion of a source and an observer. The phenomenon is named after Austrian physicist Christian Doppler, who proposed it in 1842. It applies to all waves, including sound and light, though the interpretation differs for electromagnetic radiation due to relativity.

In acoustics, the observed frequency increases when the source or observer moves toward the other, and decreases when moving apart. A common representation for a moving source and stationary medium is f' ≈ f(v/(v − vs)) when the source approaches and f' ≈ f(v/(v + vs)) when it recedes, with v the wave speed in the medium and vs the source speed. If the observer moves, similar shifts occur. The Doppler effect explains why a siren’s pitch changes as it passes and why frequency measures reveal motion relative to the medium.

In optics, the Doppler effect is relativistic. For light, frequency shifts depend on the relative speed with respect to the speed of light and follow formulas that incorporate time dilation. When motion is directly toward or away from the observer, the shift is called the relativistic Doppler shift; transverse motion involves the transverse Doppler effect.

Applications are wide: Dopplers underpin radar and Doppler weather radar, enable measurements of wind, velocity, or blood flow in medicine, and reveal cosmic motion through redshift and blueshift in astronomy. The concept remains foundational in both theoretical and applied physics.