Curiosity · Light and colour

Why is the sky blue?

If sunlight is white, why is the sky blue? Find out how air molecules scatter short-wavelength light more, why blue is scattered about six times more than red, and why the sky is not violet.

16 min
2 simulations
Mini quiz

Updated: 16 October 2026

Preview

The question

Sunlight is white; it carries every colour. Air is colourless: look at a glass of air and you see no colour at all. So why do you see a blue sky when you look up? And why does an astronaut standing on the Moon see a pitch-black sky in broad daylight?

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Colour and light

Build the light of the sky

First watch how the red, green and blue light particles are scattered in the air on the scene, and look at the scattered light meter. Then set each spotlight according to that colour's share of the skylight: the strongest colour at full (4), the others in proportion to it.

Predict first

Around midday, which colours are in the light that reaches your eye from the sky?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Number line

How many times more is blue scattered?

Drag the marker or move it with the arrow keys, then press Check.

Predict first

The wavelength of blue light is two-thirds that of red. How many times more do you think blue is scattered?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Lab

Light’s path through the air

Open full page in the Lab →

Fig. 1Light’s path through the air

Drag the lamp along the rail or use the slider

60° · path 1.15×

Height of the Sun60°

Your own value

Short path: blue scatters sideways and paints the sky; the rest of the beam stays nearly white.

Path through the air1.15×1.15 times the overhead path

Colour of the SunSky overhead

Blue (450 nm) arriving78%
Red (700 nm) arriving96%
How much more blue scatters than red (700/450)⁴, the same at every height≈ 5.9×

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

As the Sun gets close to the horizon, what happens to the colour of the light reaching the observer directly?

Bluish haze: light the air scatters sidewaysSpectrum bars: light reaching the observer, wavelength by wavelengthThe numbers come from one model: Rayleigh scattering only (1/λ⁴), a sea-level optical depth of 0.097 at 550 nm, air thinning with an 8 km scale height, the Kasten–Young air-mass formula and a 5778 K solar spectrum. Dust, ozone and clouds are left out. The drawing makes the atmosphere far too thick: most of the air lies within 15 km of the ground, while the Earth’s radius is 6,371 km.

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Frequently asked

5 questions

Is the sky blue because it reflects the sea?

No. The sky above deserts and the poles is blue too. The blue comes from air molecules scattering the blue part of sunlight more strongly. The link actually runs the other way: one reason the sea looks blue is that it reflects the sky.

Is air a blue gas?

No, air is colourless. In a glass of air there is far too little scattering to notice. The blue only becomes clear through a layer of air many kilometres thick.

How much more is blue light scattered than red?

In Rayleigh scattering, the amount scattered is inversely proportional to the fourth power of the wavelength. For 450 nm blue and 700 nm red, the ratio 700/450 raised to the fourth power is about 5.9, so blue is scattered about 6 times more.

If violet light scatters even more, why isn't the sky violet?

Sunlight contains less violet than blue, and your eye is less sensitive to violet. The scattered light reaching your eye is not a single colour but a blue-heavy mix, and your brain sees this mix as light blue.

Why are clouds white and not blue?

Cloud droplets are much larger than the wavelength of light. Particles that big scatter all colours almost equally, and an equal mix of colours looks white.