An example lecture · made with Lecture Maker

Why is the sky blue?

Start with the light itself

What is sunlight made of?

violetbluecyangreenyelloworangered

White sunlight is all the colours of the rainbow together.

NASA Space Place, "Why Is the Sky Blue?", spaceplace.nasa.gov/blue-sky · checked 5 Oct 2026

Air bounces light around

Which colour does the air scatter most?

How much each colour is scattered

Scattering grows as the wave gets shorter: 1 / wavelength⁴

Blue450 nm

~5.9x

Red700 nm

1x

our calculation (700 / 450)⁴

NASA Space Place, "Why Is the Sky Blue?" (blue travels as shorter waves, scattered more) · Wikipedia, "Rayleigh scattering" (scattering ~ 1/wavelength⁴) · checked 5 Oct 2026 · en.wikipedia.org/wiki/Rayleigh_scattering

Same sun, same air, longer road

Why are sunsets red?

The sun setting over the sea: deep blue sky overhead, orange and yellow at the horizon

NoonA short path through the air

SunsetA long path: the blue is scattered out on the way

NASA Space Place, "Why Is the Sky Blue?" (at sunset more of the blue is scattered, reds and yellows pass through) · Wikipedia, "Rayleigh scattering" · checked 5 Oct 2026 · spaceplace.nasa.gov/blue-sky · Photo: Aleksandr Aleksandrov, "Sea Sunset", CC0, Wikimedia Commons