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Light taking the long way

Why Are Sunsets Orange?

Low sunlight crosses so much air that the blue is scattered away, leaving red.

Midday — short path Blue survives, sky isblueSunset — long path Blue scattered out, skyis orange
Same sunlight. Much more air in the way.

  1. Step 1 of 5

    At sunset the Sun's light comes in sideways.

    Sun at the horizonabout 40× moreairSun overheadshortest path

    Overhead, light crosses the atmosphere by the shortest route. Near the horizon it slices along it, travelling far further through air.

  2. Step 2 of 5

    More air means more scattering.

    Low sunlight Blue scatteredaway along the way

    Air scatters blue light aside. Given forty times as much air to cross, nearly every bit of blue gets knocked out of the beam.

  3. Step 3 of 5

    What survives is red and orange.

    Blue — short waves Scattered outRed — long waves Gets through

    Long red waves slip past air molecules without being deflected. They are the only part of the beam that makes it all the way to you.

  4. Step 4 of 5

    Clouds light up because they catch the leftovers.

    1 Only red lightremains2 Clouds bounceit back down3 You see red andgold clouds

    Clouds scatter whatever light reaches them. At sunset the only light left is red, so the clouds glow red and gold from underneath.

  5. Step 5 of 5

    Dust and haze push it further.

    Clean air Gentle orangeDusty air Deep, burning red

    Extra particles scatter even more, deepening the colour. This is why the most spectacular sunsets follow big fires or volcanic eruptions.

    Sunrises use exactly the same physics — mornings just tend to have less dust stirred up in the air.

The short version

Low sunlight crosses far more air, which scatters the blue away and leaves red and orange to reach you.

Try it yourself

Compare a sunset over a city with one over open countryside. The dustier city air usually gives the richer colour.

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