Why is the sky blue, and sunsets red? (for a curious adult who has never studied physics). Cover, in this order: What is white light made of?; What does air do to a beam of light?; Why does blue light scatter more than red?; So why isn't the sky violet?; Why are sunsets red?; Why are clouds white? — page 3: Blue Scatters Most

Nib asked for

Blue Scatters Most

the short answer

Air scatters every color, but the shorter the wavelength, the far more strongly it scatters, so blue is redirected much more than red.

4 steps, drawn by Nib. Read it here, or watch him draw it. Updated 16 Oct 2026.

1two waves

Light travels as a wave, and each color has its own wavelength, the distance from one crest to the next. Red waves are long, about 700 nanometers. Blue waves are short, about 450. The air molecules that scatter sunlight are far smaller than either one, which will matter in the next step.

Each color is a wave; red is long, blue is short.

2scattering

When light meets a molecule, the wave shakes the electric charges inside it back and forth, and the shaking molecule sends light out in every direction. That re-sent light is the scattering from page 2. The faster the wave wiggles, the harder it shakes the molecule, and the more strongly the molecule re-radiates.

A light wave shakes a molecule, and the shaking molecule sends light out sideways.

3blue wins

Shorter waves wiggle faster, so a tiny molecule scatters them much more strongly. The effect is steep: halve the wavelength and the scattering is about sixteen times stronger. Blue is not twice as scattered as red but roughly five to six times, because its wavelength is about two thirds as long.

Shorter wavelength means much stronger scattering: blue is scattered about 5 to 6 times more than red.

a common mix-upRed is not left alone by air. It is scattered too, just far less than blue.

check yourself

If a color's wavelength were cut in half, would it scatter twice as much, or more?

Show the answer

Much more: about sixteen times as much, because scattering rises steeply as wavelength shrinks.

4blue sky

Now picture sunlight, white and full of every color, crossing the sky. Each molecule redirects a little of every color, but blue far more than red. So blue light gets knocked sideways and arrives at your eyes from all over the sky, while most red keeps traveling straight on. Looking away from the sun, you see that scattered blue everywhere.

Blue is bounced around the whole sky, so every patch of sky sends some blue to your eyes.

The sky is blue because air scatters short waves far more than long ones, sending blue light to you from every direction.