Scattering: Blue Skies & Red Sunsets
Air molecules scatter blue light far more than red, and that paints the sky.
Lesson at a glanceShow the key points
- Scattering: molecules absorb light and re-radiate it in other directions (unlike absorption, which turns it into internal energy).
- For particles smaller than λ, scattering ∝ 1/λ⁴ (Rayleigh): violet and blue scatter most, red least.
- Red signals are used for danger because red travels furthest with least scattering.
- The sky looks blue because we see scattered sunlight; the low sun looks red because the long path scatters the blue away.
- With no atmosphere the sky would be black (as at ~20 km height). Larger droplets and dust don't follow 1/λ⁴, giving pale or greyish skies.
Why is the sky blue and not violet, white or black? And why does the same sun turn orange at dusk? Both answers come from how strongly tiny molecules redirect different colours.
What scattering is
When a beam of light crosses a gas, some of it appears in other directions. Molecules absorb a little of the light and re-radiate it every which way. This is scattering. It differs from absorption: in absorption, light energy becomes internal energy of the medium; in scattering, the energy is sent off in new directions. Either way, the beam gets weaker.
Rayleigh’s law
How strongly light scatters depends on the size of the particles and on the wavelength. For particles much smaller than the wavelength, such as air molecules:
How much more strongly do air molecules scatter 400 nm violet light than 700 nm red light?
Red for danger
Red light is scattered least, so a red signal is seen from the furthest away through air, mist and haze. That’s why danger and stop signals are red.
The blue sky
When you look at the sky away from the sun, the light reaching your eye has been scattered towards you by air. Short wavelengths are scattered most. Violet scatters even more than blue, but sunlight contains less violet, and our eyes are less sensitive to it, so the sky looks blue.
Red sunrise and sunset
When the sun is low, its light crosses a much longer stretch of atmosphere to reach you. Along the way most of the blue and nearby colours are scattered out of the beam, and what arrives straight from the sun is mostly orange and red.
A black sky
Without an atmosphere there would be nothing to scatter sunlight towards you, so the sky would be black, with stars visible by day. About 20 km up, where the air is very thin, the sky does turn black, as high-altitude pilots and astronauts report.
Haze and pale skies
Water droplets, dust and smoke particles also scatter light. Many are larger than the wavelength, and for them the 1/λ⁴ law doesn’t hold: they scatter all colours more evenly. So a humid sky before rain looks pale blue, a clear dry sky deep blue, and the sky over smoky industrial cities greyish and hazy until rain or wind clears the particles.
- 1Scattering: molecules absorb and re-radiate light in other directions (not converting it to internal energy).
- 2For particles smaller than λ, scattering ∝ 1/λ⁴: violet and blue most, red least.
- 3Red signals carry furthest; the sky is blue; the low sun looks red.
- 4No atmosphere, no scattering: a black sky.
- 5Larger particles scatter all colours: pale, white or hazy skies.
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Keep exploring
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