Doppler Effect
Move source — see red/blueshift.
An ambulance with its siren on is driving straight toward you at high speed. As it passes you and drives away, you notice a distinct change in the siren’s pitch.
How does the pitch change as the ambulance passes you?
The pitch suddenly drops from high to low! While approaching, the ambulance compresses its sound wavefronts ahead of it — you hear a higher pitch. The instant it passes and starts receding, the wavefronts behind it are stretched — you hear a lower pitch. The actual siren frequency never changes; only your PERCEIVED frequency does.
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Watch the wavefronts compress in front of the moving source and stretch behind it. Listen (visually!) as the observer receives higher frequency when the source approaches and lower when it recedes.
f′ = f₀ × (v + v_obs)/(v - v_src)
Source approaching: f′ > f₀ (higher pitch). Source receding: f′ < f₀ (lower pitch). At v_src = v: singularity → sonic boom!
The classic example: an ambulance siren sounds higher pitched when approaching and suddenly drops in pitch after passing. The actual siren frequency never changes.
Edwin Hubble found that distant galaxies’ light is redshifted — stretched to longer wavelengths. This proved the universe is expanding, with more distant galaxies receding faster.
Police radar measures the Doppler shift of reflected radio waves to determine car speed. The faster the car, the larger the frequency shift.
Doppler ultrasound measures blood flow speed by detecting the frequency shift of reflected sound waves. It can map blood flow through the heart in real time.
“The Doppler effect turns every wave into a speedometer. It tells us how fast ambulances move, how fast galaxies recede, and how fast blood flows through your heart.”
Doppler Lab
Control a moving wave source and observe the Doppler effect from any position. Adjust source speed, wave speed, and frequency. See wavefront compression and stretching, and witness the sonic boom when the source exceeds the wave speed!
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Start with a slow-moving source. Note how wavefronts compress slightly ahead and stretch slightly behind. Now increase the speed toward the wave speed — watch the wavefronts pile up in front. At Mach 1, you get a shock wave! Go supersonic and see the Mach cone form. Measure the cone angle and verify sinθ = v/v_src.
The Doppler effect is universal: it works for sound, light, water waves — any wave. For light, it explains redshift and blueshift. For sound, it explains siren pitch changes. And when v_source > v_wave, you get a shock wave — a sonic boom for sound, Cherenkov radiation for light.