Effet Doppler
The pitch of a passing siren drops as it goes by, and the reason is visible once you draw the wavefronts. This simulation shows them bunching ahead of a moving source and stretching out behind it.
Comment utiliser cette simulation
- Move the source and the observer independently
- Change the source speed, including beyond the wave speed
- Display the wavefronts and the observed frequency
Ce qu’il faut observer
- Wavefronts bunch ahead of the source and spread out behind
- The observed pitch drops abruptly as the source passes the observer
- At source speeds above the wave speed, the fronts pile into a shock cone
La physique derrière
A moving source emits each wavefront from a slightly different place, compressing the wavelength ahead and stretching it behind. The observed frequency is f' = f(v ± v_o)/(v ∓ v_s). The shift depends on relative motion along the line joining source and observer, which is why the pitch changes most sharply as the source passes closest.