The Doppler Effect

Why sirens change pitch

An interactive lesson with 7 steps, about 6 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• A moving source squeezes the waves ahead of it (higher frequency) and stretches the waves behind it (lower frequency).
• $f' = f\,v/(v \mp v_s)$ for a source approaching (−) or moving away (+).
• Faster than sound, the wavefronts pile up into a sonic boom. For light, the same effect gives redshift.

Steps

1. Passing siren
2. Bunching up
3. The formula
4. Approaching
5. Moving away
6. Sound barrier
7. Redshift