Skip to content
Know.itKnow.it
Create an animated lesson
On request

This article is free to read. Creating a lesson uses your daily or stored credits; review the language and settings before starting.

Why is the sky blue?

ScienceLesson on requestFree to read

Quick answer

Sunlight is a mix of colors. As it passes through the atmosphere, tiny air molecules scatter short-wavelength blue light much more than red light. This scattered blue light bounces in all directions and reaches our eyes from across the sky, making it appear blue.

The sky looks blue because air molecules scatter short-wavelength blue sunlight much more strongly than longer wavelengths, sending blue light toward our eyes from all directions. This wavelength-dependent scattering is called Rayleigh scattering.

Step by step

  1. Sunlight enters Earth's atmosphere containing all visible wavelengths.
  2. Air molecules absorb and re-emit light in random directions.
  3. Short wavelengths like blue are scattered far more efficiently than red.
  4. Scattered blue light travels from all parts of the sky to the observer.
  5. The eye combines this scattered light and perceives a blue sky.

Key things to understand

  1. Sunlight contains all visible colors, each with a different wavelength.
  2. Air molecules are far smaller than the wavelength of visible light.
  3. Small particles scatter short wavelengths more efficiently than long ones.
  4. Violet-blue light is scattered roughly five to six times more strongly than red light by air molecules.
  5. Scattered blue light reaches our eyes from every part of the sky.

Labelled diagram

  1. Sunlight

    White light from the Sun contains all visible colors.

  2. Atmosphere

    Air molecules are tiny compared to the wavelength of light.

  3. Scattering

    Blue light is scattered much more than red light.

  4. Blue sky

    Scattered blue light comes from all directions to your eyes.

How blue light reaches your eyes

Real-world examples

  • On a clear day in Delhi, the sky appears a deep blue because of strong Rayleigh scattering.
  • At sunset over the Arabian Sea, the sky turns orange as blue light is scattered away.
  • In the thin air of the Himalayas, the sky looks a darker blue because there are fewer molecules to scatter light.
  • Photographs taken on the Moon show a black sky because there is no atmosphere to scatter sunlight.

Common mistake

✗ Myth: The sky is blue because it reflects the blue color of the oceans.

✓ Fact: The sky is blue because air molecules scatter blue sunlight far more than red light. The ocean looks blue partly for the same scattering reason, not the other way around.

People also ask

What causes the sky to be blue?
Sunlight is made of many colors. When it passes through the atmosphere, tiny gas molecules scatter the shorter blue wavelengths much more than the longer red ones. This scattered blue light reaches our eyes from every direction, so the sky looks blue.
Why is the sky blue and not green?
Green light has a longer wavelength than blue, so air scatters it less. Violet is scattered even more than blue, but sunlight contains less violet, some is absorbed high in the atmosphere, and our eyes are more sensitive to blue, so the scattered skylight looks blue.
Why is the sky blue during the day but black at night?
During the day, sunlight scatters off air molecules and fills the sky with blue light. At night, the Sun is below the horizon, so no sunlight enters our atmosphere to scatter. Without scattered light, the sky appears black.
Why is the sky not violet if violet scatters even more?
Violet light does scatter more than blue, but the Sun emits less violet light and our eyes are less sensitive to it. The combined effect means we perceive the scattered light as blue rather than violet.
Why does the sky turn red at sunset?
At sunset, sunlight passes through more atmosphere, scattering away most blue light. The remaining light that reaches our eyes is dominated by longer red and orange wavelengths.
Is the sky blue on other planets?
It depends on the atmosphere. Mars has a thin, dusty atmosphere that makes its sky butterscotch or pink. The gas giant planets have different compositions and can appear blue or other colors.
Why do astronauts see a black sky?
In space there are no air molecules to scatter sunlight, so no scattered light reaches the eyes from empty directions. The sky appears black except where the Sun or other bright objects are directly visible.

Part of the guide: Everyday Science: Light, Heat & Matter →