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Light & Colour · Question 02 of 20

Why are sunsets red?

A sunset is the sky-blue story told backwards: it is what is left of sunlight after the blue has been stolen.

Plate I — The colour of the sky Rayleigh · 1/λ⁴ · Kasten–Young airmass
Drag the sun from overhead down to the horizon and watch the answer happen.
Midday
FIG. I — RAYLEIGH SCATTERING
Sun altitude 62° above horizon
Or just drag the sun across the sky.
Air the light crosses
1.1× noon
Blue scattered away
35%
What reaches your eye, by colour
violetred
Sunlight looks white but it's secretly every colour mixed together. Blue light is the bounciest, so it ricochets off the air all over the sky — that's why the sky looks blue. Drag the sun down low and the light has to travel through so much more air that all the blue bounces away before it reaches you. Only the warm reds get through — a sunset.
The short answer

At sunset the light skims sideways through far more air. All the bouncy blue gets scattered away before it reaches you, so only the warm reds and oranges survive the trip.

What's actually happening

At noon, sunlight punches straight down through the atmosphere — the shortest possible path. At sunset, the geometry turns brutal: light has to skim sideways through the air, a path up to thirty-eight times longer before it reaches your eye.

Scattering is a toll collected per kilometre of air, and it charges blue light five times the rate of red. Over the short noon path, blue pays a small toll and mostly gets through. Over the long sunset path it is bankrupted — first violet, then blue, then green are scattered away to colour someone else's daytime sky. What survives the journey to your eye is the leftovers: orange and red.

This is also why the most lurid sunsets follow volcanic eruptions or arrive on dusty evenings. Fine aerosols high in the atmosphere add extra scattering exactly where the sunset path is longest, taking out even more of the short wavelengths and pushing the survivors deeper into red.

Try it at home Sunset in a glass, part two
  1. 1Use the milk-in-water glass from the sky-blue experiment, but keep adding milk, a drop at a time.
  2. 2Shine the torch through the long side of a rectangular container if you have one — the longer the path, the better.
  3. 3Watch the transmitted beam go from white to yellow to orange to red as you add "atmosphere". You are replaying the last ten minutes before sunset.