Why Is the Sky Blue? Rayleigh Scattering Explained
Why the sky is blue, explained simply: how sunlight scatters off air molecules, why short wavelengths scatter more, red sunsets and why it isn’t violet.
The Doppler Effect Explained
The Doppler effect explained: why a passing siren changes pitch, the formula, redshift and blueshift of light, and uses in radar, medicine and astronomy.
Key takeaways
- Moving sources squash waves ahead of them and stretch waves behind them.
- Approaching sounds have higher pitch; receding sounds have lower pitch.
- For light, this appears as blueshift and redshift, which astronomers use to measure motion.
On this page
When an ambulance races past, its siren sounds higher as it approaches and lower as it moves away. The siren doesn’t change; the motion changes the waves reaching you. This is the Doppler effect.
How it works
A moving source emits waves while travelling. Ahead of the source, wave crests are bunched together: shorter wavelength, higher frequency. Behind it, they’re spread out: longer wavelength, lower frequency.
The sound formula
For a source moving towards or away from a stationary listener:
f′ = f × v ÷ (v ∓ vₛ)
- f = emitted frequency
- v = speed of sound (about 343 m/s in air at 20°C)
- vₛ = source speed (minus when approaching, plus when receding)
Worked example
A siren at 700 Hz approaches at 30 m/s: f′ = 700 × 343 ÷ (343 − 30) ≈ 767 Hz. As it moves away: f′ = 700 × 343 ÷ (343 + 30) ≈ 644 Hz.
Light: redshift and blueshift
Light waves shift too. Objects moving away appear slightly redder (redshift); objects approaching appear bluer (blueshift). Astronomers use this to measure how galaxies move, which revealed that the universe is expanding.
Everyday uses
- Police speed radar and weather radar
- Doppler ultrasound to measure blood flow
- Astronomy, to detect planets around other stars by their star’s wobble
Related topics
Light’s behaviour also explains why the sky is blue and how rainbows form.
Frequently asked questions
Does the Doppler effect happen if the listener moves?
Yes. Motion of the listener also changes the observed frequency.
Why don’t we notice the Doppler effect with light every day?
Everyday speeds are tiny compared with the speed of light, so the shift is extremely small.
What is redshift?
The stretching of light to longer wavelengths when a source moves away from the observer.
Sources
Every article is edited by a human and checked against our editorial policy. Spotted a mistake? Tell us.
Keep reading
How Rainbows Form
How rainbows form: refraction, dispersion and reflection inside raindrops, why the colours appear in order, why rainbows are arcs and double rainbows.
Snell’s Law and Refraction
Refraction and Snell’s law explained: why light bends between materials, refractive index, n₁ sin θ₁ = n₂ sin θ₂, total internal reflection and a calculator.