Why Is the Sky Blue?

Every day we look up and see a blue sky — but why blue and not red or green? The answer involves the physics of light, the tiny molecules in the atmosphere, and a phenomenon called Rayleigh scattering.

What Is Light?

Sunlight looks white, but it is actually a mixture of all the colours of the rainbow — red, orange, yellow, green, blue, indigo, and violet. Each colour corresponds to a different wavelength of light.

Red light has a long wavelength (about 700 nm), while blue and violet light have much shorter wavelengths (around 400–450 nm). This difference in wavelength is the key to the blue sky.

Fun fact: 1 nanometre (nm) is one billionth of a metre. A human hair is about 80 000 nm wide. Light wavelengths are incredibly tiny!

What Is the Atmosphere?

The Earth is surrounded by a thin layer of gas called the atmosphere. Most of it (78 %) is nitrogen (N₂) and about 21 % is oxygen (O₂). These molecules are incredibly small — far smaller than the wavelength of visible light.

When sunlight enters the atmosphere, it collides with these tiny molecules. The molecules absorb the light for a brief moment and then re-emit it in a random direction. This is called scattering.

Rayleigh Scattering

Not all colours scatter equally. The British physicist Lord Rayleigh (John William Strutt) showed in 1871 that the amount of scattering depends strongly on the wavelength of light. Shorter wavelengths scatter much more than longer ones.

The mathematical relationship is: scattering intensity is proportional to 1 / λ⁴, where λ (lambda) is the wavelength.

Example: Blue light (λ ≈ 450 nm) scatters about (700/450)⁴ ≈ 5.5 times more than red light (λ ≈ 700 nm). So for every red photon scattered, roughly 5–6 blue photons are scattered.

Because blue light is scattered in all directions across the entire sky, when you look anywhere in the sky (away from the Sun), you see blue light that has been scattered towards your eyes. That is why the sky is blue!

Violet light actually scatters even more than blue, so why isn't the sky violet? Two reasons: the Sun emits less violet light to begin with, and our eyes are less sensitive to violet than blue.

Why Are Sunsets Red?

At sunrise and sunset, sunlight travels through a much thicker layer of atmosphere to reach your eyes. After all that extra scattering, almost all the blue light has been scattered away in other directions. What remains is the longer-wavelength light: red, orange, and pink.

The same physics that makes the midday sky blue also makes sunsets glow with warm colours — it just depends on how much atmosphere the light has travelled through.

Why are some sunsets more spectacular? Dust particles, smoke, and aerosols in the atmosphere add extra scattering. After large volcanic eruptions, sunsets can turn dramatically red and purple for months.

The Simple Maths

Rayleigh's scattering formula for the intensity I of scattered light from a single molecule is:

I ∝ (1 + cos²θ) / λ⁴

Here θ is the angle between the incoming and outgoing light direction, and λ is the wavelength. The λ⁴ in the denominator is the key: halving the wavelength increases scattering by a factor of 2⁴ = 16!

The total amount of blue light scattered across the sky is the result of billions upon billions of such individual scattering events every second.

Try It Yourself

You can see Rayleigh scattering at home with a glass of water and a few drops of milk. Add just 2–3 drops of milk to a glass of water; when you shine a torch through the side, the water looks blue. When you look through the glass at the torch (like looking at a setting sun), it looks orange or red.

The tiny fat droplets in milk play the role of atmospheric molecules.
Short wavelengths (blue) scatter sideways — the glass looks blue.
Long wavelengths (red/orange) travel straight through — just like a sunset.

Explore further: Try the Rainbow simulation and the Atmospheric Scattering simulation to see these principles in 3D real time.

🌍 Open Atmosphere →