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.
Most people think sunsets are red because the cooling sun changes colour or burns differently in the evening. In fact the sun is unchanged; its light just crosses far more air, which scatters away the blue and leaves red behind.
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.
A sunset is the leftover light: the long sideways path through the air strips out the blue first, so only warm reds survive the trip to your eye.
- 1Use the milk-in-water glass from the sky-blue experiment, but keep adding milk, a drop at a time.
- 2Shine the torch through the long side of a rectangular container if you have one — the longer the path, the better.
- 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.
Common questions
Fine aerosols high in the atmosphere add extra scattering exactly where the sunset path is longest. They remove even more of the short wavelengths, pushing the surviving light deeper into red.
The thick air refracts the lower edge of the sun upward more than the top edge. This uneven bending visibly flattens the disc in the last moments before it sets.
During an eclipse, the only sunlight reaching the Moon has been bent through Earth's atmosphere, through every sunrise and sunset on the planet at once, so the Moon glows with that filtered red light.