Demo deck

Why is the sky blue?

Rayleigh scattering, in six slides

dcoe.uk · November 2026

The puzzle

Sunlight is white. Why isn’t the sky?

White light is a mix of every colour, from violet to red.

400 nm
violet700 nm
red

Air is mostly nitrogen and oxygen molecules, about a thousand times smaller than a light wave.

So which colours does air scatter towards us, and why?

Midday: blue overhead
Sunset: orange and red

Rayleigh scattering

Shorter wavelengths scatter much more

scattering angle
distance away
refractive index
molecule size

For air, only the wavelength changes

Which means

Halve the wavelength and the scattering goes up 16 times.

Try it

Blue scatters about six times more than red

450 nm

5.9×

as much scattering as 700 nm red light

Drag the slider, or click the chart.

Interactive in the web version.

Sunsets

At sunset, light crosses far more air

Atmosphere Earth High sun Low sun about 38× more air blue scattered out You
Not to scale: the atmosphere is drawn far thicker than it is.
  1. High sun: light takes the shortest path down through the air.
  2. Low sun: the light passes through about 38 times as much air.
  3. Most of the blue is scattered out on the way. Orange and red get through.

One last question

So why isn’t the sky violet?

1

Less violet to start

Sunlight contains less violet than blue.

2

Eyes barely see violet

The eye’s colour sensors respond only weakly to violet.

3

The eye mixes the rest

Sky light is mostly blue and violet, with some green and red. We see that mix as pale blue.

TakeawayTiny molecules scatter short wavelengths most: . Blue skies and red sunsets are the same effect.