Why is the sky blue, and why is sunset orange?

Short answer

When sunlight hits the molecules in the air, short-wavelength blue light changes direction far more than long-wavelength red light, and it reaches your eyes from every part of the sky. At sunset, though, light travels a much longer path through the atmosphere; most of the blue scatters away along the path, and what reaches you is mostly orange and red.

Short wavelengths scatter more

The air itself is invisible, but it is not empty: it is full of nitrogen and oxygen molecules. These molecules are thousands of times smaller than the wavelength of visible light. When sunlight hits them, part of it keeps traveling in its original direction, and part of it scatters in every direction. This scattering is the only reason the sky looks bright: the light that reaches your eyes from every point you are not looking directly at the sun is sunlight that has scattered in the air.

The key point is that this scattering does not treat every wavelength equally. For very small particles, the strength of scattering is inversely proportional to the fourth power of the wavelength. That may sound abstract, but the result is striking: 450-nanometer blue light scatters roughly six times more strongly than 700-nanometer red light. The sun sends out white light, but the share that scatters into the rest of the air is weighted toward blue. Whichever direction you turn your head, the scattered light coming from that direction is blue.

Why clouds are white and fog is gray, though, comes down to a different kind of scattering — the scattering of larger particles. The distinction between the two types, and why milk is opaque, is covered in BI-05.

The long path of light at sunset

At midday, when the sun is overhead, its light crosses the atmosphere by the shortest path. As the sun approaches the horizon, that path lengthens, because the light now has to travel through the air sideways instead of straight down. The longer the path, the more molecules the light encounters, and it loses a little more blue at every encounter. The light that reaches you from a sun near the horizon has left most of its blue behind along the way; what remains is orange and red.

That is why two things happen at once at sunset. The sun's disk itself turns from yellow to orange, then to red. The clouds around the sun are lit by this reddened light too, so they take on shades of pink, orange, and crimson. When particles such as dust, smoke, or sea salt increase in the air, the light is filtered even further; this is one reason the sun can look an unusually deep red after large forest fires.

The same shift shows up in the light that falls on human skin. Light from a low sun makes every skin tone look warmer and more orange; this is what photographers call the "golden hour." What changes is not the skin but the light hitting it. Your brain notices this shift to some degree and tries to correct for it; when that correction works and when it misleads you is the subject of KE-03.

Why isn't the sky violet?

If the rule says shorter wavelengths scatter more, and violet light is even shorter than blue, why isn't the sky violet? Several reasons work together here. The sun sends out less violet light than blue to begin with. Your eye is also less sensitive to this end of the spectrum. On top of that, the light coming from the sky is not a single wavelength but a mixture weighted toward blue while still carrying a bit of every color.

In short, the color of the sky is not only a matter of physics but also of your eyes. The same sky looks different to a bird or a bee with different cone types; KE-06 looks at this difference.

Example — Blue sunset on Mars

On April 15, 2015, NASA's Curiosity rover captured a sunset in color at Gale Crater on Mars. In the image, the area around the sun looked bluish. This scene looks like the reverse of Earth's, but the same physics is at work — only the particles are different. In Mars's thin atmosphere, what fills the air is not small gas molecules but fine dust grains. The size of this dust lets blue light travel more easily toward the sun's direction, while yellow and red scatter across the sky at wider angles. The result: a yellowish-brown sky by day, and a blue halo around the sun at sunset.

This example shows that the color of a sky does not depend on the planet itself, but on the size of the particles in its atmosphere and the length of the light's path.

Common misconception

Summary

1. Molecules in the air scatter short-wavelength light far more strongly; the scattered light reaching you from every part of the sky is weighted toward blue.
2. At sunset, light travels a much longer path through the atmosphere, loses its blue along the way, and reaches you as orange and red.
3. The sky looking pale blue rather than violet comes from both the content of sunlight and the sensitivity of your eyes.

Test yourself

1. (Easy) Which color of light do molecules in the air scatter more strongly: blue or red?
2. (Easy) How does the path of sunlight through the atmosphere at sunset differ from its path at midday?
3. (Medium) Why is the sky black on the Moon?
4. (Medium) Why might the sun look redder after a large forest fire?
5. (Hard) Even though violet light scatters more than blue, why don't we see the sky as violet?
6. (Hard) Does the blue appearance around the sun at Martian sunsets mean Earth's rule is being broken? Justify your answer.

Answers

1. Blue; short wavelengths are scattered far more strongly by small particles, with 450 nm light scattering roughly six times more than 700 nm light ("Short wavelengths scatter more").
2. It lengthens; as the sun approaches the horizon, light travels through the air sideways and encounters more molecules ("The long path of light at sunset").
3. The Moon has no atmosphere to scatter light. Sunlight reaches the surface directly, but no scattered light comes to the eye from any point in the sky; this is why the sky is dark even in daytime ("Short wavelengths scatter more").
4. Smoke particles in the air filter the light's path even further; even more blue is lost, and the light reaching you from the sun is weighted toward red ("The long path of light at sunset").
5. Sunlight contains less violet than blue, your eye is less sensitive to violet, and sky light is a mixture carrying a bit of every color; the eye perceives this mixture as a pale blue ("Why isn't the sky violet?").
6. No. The rule is the same: scattering depends on particle size and wavelength. On Mars, scattering is done by fine dust rather than gas molecules, and the size of that dust lets blue travel more easily toward the sun ("Example — Blue sunset on Mars").

Sources

• Strutt, J. W. (Lord Rayleigh), "On the Light from the Sky, its Polarization and Colour," Philosophical Magazine, February 1871.
• Smith, G. S., "Human color vision and the unsaturated blue color of the daytime sky," American Journal of Physics 73(7): 590–597, 2005.
• NASA/JPL-Caltech/MSSS/Texas A&M Univ., "Sunset in Mars' Gale Crater," photo archive PIA19400, 2015.

Next step

BI-05 separates the two types of scattering: it connects particle size to wavelength and explains why clouds are white, why milk is opaque, and why blue eyes contain no blue pigment. KE-03, meanwhile, moves to what happens in the brain with the color shift at sunset: it explains why the same photograph evokes different colors in different people, through the brain's estimate of the color of the light.