Designed by Fu-Kwun Hwang and remixed by lookang. This version keeps the motion in one dimension so learners can focus on how relative motion changes the spacing of wavefronts and the frequency received by the observer.

# Doppler Effect in 1D

The red source emits wavefronts at a constant period T. The green observer receives them at a different rate when the source, the observer, or both move relative to the medium.

- Start with a preset, then compare the space view and the timing graph.
- In the space view, look for compressed wavefronts in front of the source and stretched wavefronts behind it.
- In the timing graph, red bars mark emission times and green bars mark detection times.
- If the green bars are closer together than the red bars, the detected frequency f' is higher than the emitted frequency f. If they are farther apart, f' is lower.
- Use Step to pause and reason about one moment at a time. Drag the source or observer only while paused.
- The sonic-boom presets are extension cases. In that regime, compare the shock-wave envelope instead of the usual subsonic Doppler formula.

Suggested inquiry: Which changes the received frequency more clearly in this model, moving the source or moving the observer by the same fraction of the sound speed?