The 90-second version
- Sunlight appears white but contains all colours of the rainbow — each colour is a different wavelength of light
- As sunlight enters the atmosphere, air molecules (mainly nitrogen and oxygen) scatter some of it in all directions
- Blue light has a shorter wavelength than red light and gets scattered much more strongly — about 10× more than red
- So from anywhere you look in the sky (except directly at the sun), you see this scattered blue light
- At sunset, the sun is near the horizon — sunlight has to travel through much more atmosphere to reach you
- By then, most of the blue has been scattered away sideways, leaving the longer-wavelength red and orange to come straight through
Light is a wave — and wavelength determines colour
Visible light is electromagnetic radiation with wavelengths between roughly 380 nm (violet) and 700 nm (red). Your eye has three types of cone cells tuned to short (blue), medium (green), and long (red) wavelengths. The mix of signals from these cones creates your perception of colour.
Sunlight is (approximately) a mix of all visible wavelengths in roughly equal amounts, which your brain interprets as white.
What happens when light hits a molecule
An air molecule — nitrogen (N₂) or oxygen (O₂) — is much smaller than the wavelength of visible light. When a photon strikes it, the electric field of the light wave briefly displaces the molecule’s electron cloud, creating a tiny oscillating dipole. This dipole radiates electromagnetic energy in all directions — that’s scattering.
Rayleigh scattering (named after Lord Rayleigh, who published the theory in 1871) describes how small particles scatter light. The critical result is:
Scattering intensity ∝ 1/λ⁴
The scattering is inversely proportional to the fourth power of wavelength. This means:
- Blue light (450 nm) scatters approximately 5.5× more than red light (700 nm)
- Violet light (380 nm) scatters even more than blue
So in precise numbers: red light (700 nm) vs blue (450 nm):
(700/450)⁴ = 1.56⁴ ≈ 5.9×
Blue scatters about 6× more than red. Violet scatters even more.
So why isn’t the sky violet?
If violet scatters the most, shouldn’t the sky be violet?
Two reasons:
- Sunlight has less violet in it. The sun emits less violet than blue — it’s a G-type star whose emission spectrum peaks in the green/yellow range.
- Your eyes are less sensitive to violet. Human cone cells (specifically S-cones) respond to blue more strongly than violet.
The combination means our brains perceive the sky as blue rather than violet, even though violet is scattered as much or more.
Why sunsets are red and orange
At noon, the sun is overhead. Sunlight travels through roughly 100 km of atmosphere to reach you — the shortest possible path.
At sunset, the sun is on the horizon. Sunlight now travels through the atmosphere at a shallow angle, traversing perhaps 40× more atmospheric mass before reaching your eyes.
All that extra atmosphere scatters away the blue and violet light sideways to people elsewhere on Earth. What arrives at your eyes is the remainder: the wavelengths that scattered least — red and orange.
The effect is enhanced by:
- Humidity and aerosols: scatter even more blue
- Volcanic eruptions: fine ash particles in the stratosphere produce vivid red sunsets for months (El Chichón 1982, Pinatubo 1991 produced spectacularly coloured skies globally)
- Dust and pollution: larger particles cause Mie scattering (wavelength-independent), adding white/grey tones but also enriching reds
Why clouds are white (Mie scattering)
Clouds contain water droplets ~10–100 micrometres in diameter — thousands of times larger than air molecules and comparable to or larger than visible light wavelengths. At this scale, a different phenomenon takes over: Mie scattering.
Mie scattering is much less wavelength-dependent — all visible wavelengths scatter roughly equally. So clouds scatter white light as white. From below, a thick cloud blocks sunlight and appears dark grey; from an aeroplane above, it looks brilliantly white.
Why space is black
The sky is blue because the atmosphere scatters light in all directions. Above the atmosphere, there’s nothing to scatter sunlight — photons travel in straight lines. Look anywhere except directly at the sun, and you see no photons arriving — blackness.
Astronauts on the Moon (no atmosphere) see a permanently black sky even in full “daylight.” Stars are visible in the daytime — something impossible from Earth’s surface except during a total solar eclipse.
The mental model: atmosphere as a prism in reverse
A glass prism separates white light into a rainbow by refracting different wavelengths at different angles. The atmosphere acts similarly but via scattering: it separates white sunlight, sending short wavelengths (blue) bouncing in all directions while letting long wavelengths (red) travel more directly. Instead of a neat rainbow spread out in one place, the “blue” fraction is broadcast across the entire sky.
Common misconceptions
“The sky is blue because the ocean reflects blue water.” The ocean appears blue because the sky is blue — not the other way around. Open ocean far from the sky (looking straight down into it) appears dark. The reflected sky provides most of the ocean’s apparent colour.
“The sky is always bluer with less pollution.” Sometimes, but very clean mountain air can scatter more blue than slightly hazy lowland air. The Rayleigh scattering of pure air molecules is the dominant effect; a little humidity or aerosol can actually enhance sunset colours while barely affecting the daytime sky.
“Red light ‘gets through’ sunsets because it’s more energetic.” The reverse — red light has less energy (lower frequency) than blue. Red gets through because it’s less efficiently scattered, not because it’s stronger.