The Doppler Effect

Why sirens change pitch

An interactive lesson with 6 steps, about 5 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 its waves ahead and stretches them behind.
• $f' = f\,\dfrac{v}{v \mp v_s}$: minus when approaching, plus when receding.
• Faster than sound means a shock cone (sonic boom). Light shows the same effect as red- and blueshift.

Steps

1. A passing siren
2. Bunched-up waves
3. The formula
4. Approaching
5. Receding
6. Beyond sound