[Leejohnston]

## The Doppler Effect — Why Sound and Light Shift

### 1. The Equation (Sound)

Observed frequency:

f' = f (v ± v₀) ÷ (v ∓ v_s)

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### 2. What This Means

Motion changes how frequently waves arrive.

Approaching sources compress waves → higher pitch.
Receding sources stretch waves → lower pitch.

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### 3. Light Doppler Shift

For light, motion causes:
- redshift (moving away)
- blueshift (moving toward)

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### 4. Real-World Applications

- Ambulance sirens
- Astronomy (galaxies)
- Radar guns
- Medical imaging

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### 5. Common Misconceptions

- Doppler affects speed → false
- Only applies to sound → false

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### Try It Yourself

Why does a siren suddenly drop in pitch as it passes you?