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natural-science

Why the Sky Is Blue

The sky looks blue because air redirects some colors of sunlight more than others.

Imagine tossing big red balls and tiny blue beads through a crowded room. The tiny beads are knocked in more directions, so they appear all around you.

  1. White sunlight separated into red, orange, yellow, green, blue, and violet light.White sunlightRed and orangeGreenBlue and violet

    Step 1

    Sunlight carries many colors

    Sunlight looks white, but it contains the full range of visible colors.

  2. A beam of sunlight entering a layer filled with tiny air molecules.SunlightAir moleculesEarth

    Step 2

    Light meets tiny air molecules

    As sunlight enters the atmosphere, it passes countless molecules much smaller than a speck of dust.

  3. Blue light scattering widely while red light continues mostly forward.Blue lightShorter wavesScatters stronglyRed lightLonger wavesScatters less

    Step 3

    Short waves scatter more

    Tiny molecules redirect shorter blue and violet wavelengths much more strongly than longer red wavelengths.

  4. Blue light scattering from many air molecules toward a person's eyes.many directionsSunlightBlue scattersYour eyes

    Step 4

    Blue arrives from every direction

    Scattered blue light reaches your eyes from all across the sky, not just from the Sun's direction.

  5. Blue and violet light compared by sunlight amount and human eye sensitivity.BluePlenty in sunlightEyes see it wellVioletLess in sunlightEyes see it poorly

    Step 5

    Why not a violet sky?

    Violet scatters strongly too, but sunlight has less of it and human eyes are less sensitive to violet.

  6. A long sunset path scattering blue light away while red and orange reach an observer.Low SunLong air pathRed sunset

    Step 6

    Sunsets take the long way

    Near the horizon, light crosses more air. Much of the blue scatters away, leaving more red and orange to reach you.

Common trap

The sky is blue because it reflects the ocean.

The sky is blue even far from oceans. Air molecules scatter blue wavelengths of sunlight across the sky.

Quick check

Why does a sunset often look red or orange?

  1. Blue light scatters away along the longer path
  2. The Sun cools down each evening
  3. The ocean paints the clouds
Show the answer

Sunset light crosses more atmosphere, so much of its blue light is scattered away before the direct beam reaches you.

Sources

  1. Why Is the Sky Blue?UCAR Center for Science Education
  2. Why is the sky blue?Met Office