Sunlight is every color at once. Air bounces the blue part all over the sky.
☀️ The sun makes light. Sun light looks white.
💨 Air is all around you. You cannot see it.
🔵 Light bumps into air. Blue light bounces the most. It bounces all over the sky!
👀 Look up. The sky is blue. That is blue light, bouncing down to you.
What color is sunlight?
Sunlight looks white. But white light is really every color mixed together! Red, yellow, green, blue, and purple hide inside it. A rainbow shows them when rain pulls them apart.
What does air do to light?
Air is made of tiny bits, too small to see. When sunlight hits them, some light bounces off in new directions. That is called scatter. To scatter means to spread out everywhere, like dropped marbles.
Why does blue win?
Blue light scatters much more than red light. So blue bounces all around the sky. Wherever you look, blue light comes at you. That is why the sky is blue!
Why is the sunset orange?
At sunset, sunlight travels through much more air. The blue gets bounced away before it arrives. Orange and red keep going straight. So the sky by the sun glows orange, like a big peach.
White Light Is a Rainbow in Hiding
The sky is not painted blue. There is no blue stuff up there at all. The sky is blue because of what air does to sunlight, and the story starts with a secret about white light.
Sunlight looks white, but it is really every color of the rainbow traveling together. A prism or a raindrop can pull the colors apart, and that is exactly how a rainbow appears.
Air Is Full of Tiny Bouncers
Air is made of molecules (say: MOL-eh-kyools), particles so tiny that one breath holds more of them than there are grains of sand on Earth. When sunlight passes these molecules, some of the light bounces off in a new direction. Scientists call this scattering.
Blue light scatters much more easily than red light. Almost 6 times more! Red and orange light mostly travel in a straight line, but blue light gets bounced all over the sky. Wherever you look, some of that bounced blue light is heading for your eyes.
Why Not Purple?
Violet light scatters even more than blue. So why is the sky not purple? Three reasons. The sun sends out less violet than blue. Some violet is soaked up high in the air. And your eyes see blue much better than violet. Put it all together, and the sky looks sky blue.
Sunsets: The Long Way Through the Air
At noon, sunlight comes almost straight down. At sunset, it comes in sideways and crosses about 40 times more air to reach you. On that long trip, almost all the blue gets scattered away. The orange and red keep going, and that is what paints the sunset.
Try This!
Fill a clear glass with water and stir in a few drops of milk. In a dark room, shine a flashlight through the glass. From the side, the water glows faintly blue. Looking straight through at the flashlight, it looks orange. You just made a tiny sky and a tiny sunset.
The Sun Sends Every Color at Once
The sky contains nothing blue. There is no blue gas and no blue dust up there, and no reflection of the ocean either, which is the explanation most people were handed as kids and which gets the cause exactly backwards, since the sea takes much of its color from the sky and not the other way around.
The real answer is a trick that ordinary air plays on ordinary sunlight, and the trick depends on one property of light: wavelength. Light travels as waves, and the waves have a size. Red light has long waves, about 700 nanometers from crest to crest, while blue light has shorter waves, around 450 nanometers, and sunlight carries every wavelength in between, all mixed together so thoroughly that the blend looks white.
Molecules Scatter Short Waves Hardest
Air molecules, mostly nitrogen and oxygen, are far smaller than a wavelength of visible light. When a light wave sweeps past a molecule that small, the wave's electric field shakes the molecule's electrons back and forth, and electrons that shake send out a brand-new wave of their own in every direction at once, which is all that scattering really is. Short waves shake electrons harder than long waves do. The physics works out to a startling rule.
Consequently, red and orange light mostly pass straight through the atmosphere, while blue light is knocked out of the sunbeam and bounced across the whole sky, so that when you look in any direction away from the sun, what reaches your eye is scattered light, and scattered light is mostly blue. That is the entire secret.
Why Not Violet?
Violet has an even shorter wavelength than blue, about 400 nanometers, so it scatters even more, and by the rule above the sky ought to be purple, yet it is not. Three effects pile up against violet: the sun emits less of it than blue, ozone and the upper atmosphere absorb a portion of what there is, and the cone cells in your eye respond weakly to violet but strongly to blue and green, so the blend of scattered light that finally lands on your retina gets read by your brain as pale blue rather than purple.
Sunsets and the Long Path
At midday, sunlight crosses the atmosphere almost vertically, the shortest possible path. At sunset the sun sits on the horizon and its light slices through the atmosphere sideways, covering roughly 40 times more air before it reaches you, and over that long path nearly all of the blue is scattered out of the beam long before it arrives. What survives is the light that scatters least, the oranges and the reds. The same rule that makes the sky blue at noon makes it orange at dusk, and dust or smoke in the air adds larger particles that deepen the reds, which is why sunsets after a wildfire or a volcanic eruption look so dramatic.
Clouds, Haze, and Other Planets
Cloud droplets are thousands of times larger than air molecules, comparable to or bigger than a wavelength of light, and particles that size scatter every color about equally, which is why clouds look white instead of blue. Haze and humid air hold in-between particles. That is why a muggy summer sky looks pale and washed out while a dry mountain sky looks deep blue.
Mars shows the rule from the other side. Its thin atmosphere holds fine reddish dust that turns the daytime sky butterscotch, yet those same dust grains forward-scatter blue light around the sun, so Martian sunsets glow blue. On the Moon there is no atmosphere at all, and the daytime sky is black.
Think About It
If Earth's air were made of much larger particles, what color would the sky be, and what would a sunset look like? And why does a glass of water with a few drops of milk in it glow blue from the side but orange when you look through it at a flashlight?
A Question That Took Three Centuries
Leonardo da Vinci noticed in the early 1500s that wood smoke looks blue against a dark background and reddish against a bright one, and guessed that the sky's blue came from fine matter in the air lit against the darkness of space, which put him closer to the truth than anyone would get for the next 350 years. Newton showed that white light is a mixture of colors. The mechanism that selects blue from that mixture waited until the nineteenth century.
In 1869 John Tyndall filled a glass tube with fine particles and shone a beam through it: viewed from the side the beam glowed blue, viewed end-on the transmitted light was reddened, and the sky and the sunset had been reproduced on a laboratory bench. Two years later a 28-year-old physicist named John William Strutt, later Lord Rayleigh, worked out why.
Why the Fourth Power
Rayleigh treated each scattering particle as a tiny dipole driven by the oscillating electric field of the light wave. A dipole oscillating at angular frequency ω radiates power proportional to ω to the fourth, because the radiated field is proportional to the acceleration of the charge, acceleration in simple harmonic motion carries a factor of ω squared, and squaring the field to get intensity doubles the exponent again. Fourth power of frequency, which is one over the fourth power of wavelength.
Two features of this formula matter. The wavelength dependence is steep, so 450 nm blue is scattered (700/450)4 ≈ 5.9 times more than 700 nm red and 400 nm violet about 9.4 times more, and the angular factor (1 + cos²θ) means scattering is strongest forward and backward and weakest at right angles to the beam, though it never falls to zero.
Sunset Arithmetic
The fraction of light that survives a path through the atmosphere without being scattered is exp(−τm), where τ is the optical depth at the zenith and m is the air mass, the path length relative to vertical. Rayleigh optical depth at sea level is about 0.22 at 450 nm and about 0.037 at 700 nm.
At noon, with m near 1, about 80 percent of blue light and 96 percent of red still reach the ground unscattered, so the direct sun looks only slightly yellow. On the horizon m reaches about 38, and now exp(−0.22 × 38) is roughly 0.0002 for blue while exp(−0.037 × 38) is roughly 0.25 for red, which means that blue is essentially gone and a quarter of the red remains. That asymmetry is the sunset. The sun has not changed at all.
Molecules, Not Dust
Rayleigh originally assumed the scatterers were fine dust or water droplets. In 1899 he returned to the problem and argued that air molecules alone could account for the observed brightness of the sky, with no dust required, and the final piece came in 1910, when Albert Einstein derived scattering from the statistical density fluctuations of a gas and showed that the strength of sky scattering is fixed by the number of molecules per unit volume. Measurements of sky brightness therefore give an independent estimate of Avogadro's number. They agreed with the values from other methods. The blue of the sky is, in a real sense, evidence that atoms exist.
Other Skies
Mie scattering, the regime where particles are comparable to or larger than the wavelength, is nearly color-neutral, which is why clouds are white and hazy skies are pale. Mars combines both regimes: fine iron-oxide dust reddens the daytime sky to a tan color while forward-scattering blue light near the sun, producing the blue sunsets that the Curiosity rover photographed in 2015, and Titan's thick organic haze gives that moon an orange sky instead. On any airless world the daytime sky is black. The Apollo photographs show it.
Why Your Kid Asked
"Why is the sky blue?" is the question parents dread, partly because the honest answer is a paragraph of physics and partly because most of us were handed a wrong one as children. The sky does not reflect the ocean, and the air is not blue. The correct explanation is compact, surprising, and, once you have it, explains sunsets, white clouds, blue smoke, and the black lunar sky in one go.
The Mechanism in One Paragraph
Sunlight is a broad mixture of wavelengths, air molecules are roughly a thousand times smaller than those wavelengths, and a particle that small scatters light with an intensity proportional to the inverse fourth power of wavelength (Rayleigh, 1871), so blue at 450 nm is scattered about 5.9 times more strongly than red at 700 nm. Away from the sun, everything you see is scattered light, so the sky is blue. Toward a low sun you see what is left after scattering, so sunsets are red. That is the whole thing. The rest is detail, and the detail is where it gets good.
Why Not Violet, and Why "Sky Blue" Is Unsaturated
Violet scatters even more than blue, and measured sky spectra do rise toward the violet end. Three effects keep the perceived color from being purple: the sun's output falls off in the violet relative to blue, ozone absorbs in the near-ultraviolet and violet (the Huggins bands), and human color vision weights the result, since the short-wavelength cones peak near 445 nm, the medium and long cones contribute green and red, and the brain integrates a broad, violet-leaning spectrum into a pale, unsaturated blue. Glenn Smith's analysis in the American Journal of Physics (2005) walks through this with actual cone sensitivities, and it is the clearest treatment I know.
The Sunset, Quantified
Rayleigh optical depth at sea level is about 0.22 at 450 nm and 0.037 at 700 nm (Bodhaine et al., 1999), and transmission along a slant path is exp(−τm), where m is the relative air mass. Overhead, m ≈ 1 and both colors mostly get through. At the horizon m ≈ 38, and blue transmission collapses to about 0.02 percent while red retains about 25 percent, which is the entire difference between noon and dusk. Aerosols add extinction of their own: dust, sea salt, smoke, and volcanic sulfate all scatter and absorb with their own wavelength dependence, which is why sunsets after large eruptions such as Pinatubo in 1991 were unusually red and stayed that way around the world for months.
Dust, Molecules, and Avogadro
Rayleigh's 1871 papers assumed suspended particles. In 1899 he showed that the molecules of air themselves suffice, and in 1910 Einstein derived the scattering from thermodynamic density fluctuations, tying its magnitude to the number density of molecules and turning sky brightness into one of the early independent measurements of Avogadro's number, part of the body of evidence that settled the reality of atoms in the decade after 1905. Marian Smoluchowski had reached the fluctuation picture in 1908. Einstein's paper supplied the quantitative link to the refractive index.
Clouds, Haze, and Mars
When scatterers are comparable to or larger than the wavelength, Mie scattering takes over and the wavelength dependence largely vanishes, so cloud droplets and most haze scatter white, and a humid or polluted sky looks pale for the same reason: more large particles, more white light mixed in with the molecular blue. Mars is the instructive counterexample. Its fine iron-oxide dust absorbs blue and scatters red across most of the sky, giving the familiar butterscotch daytime color, but the same micron-scale grains preferentially forward-scatter blue light into a halo around the sun, so Martian sunsets are blue. Curiosity photographed one from Gale Crater on April 15, 2015.
Two Misconceptions Worth Correcting
First, the ocean. The sea is blue mostly because water absorbs red light more than blue over a path of a few meters, with sky reflection as a secondary contribution, so the sky is not blue because of the sea. Second, "the atmosphere is blue." Air has no intrinsic color at everyday thicknesses; look through a few meters of it and it is transparent, and it takes many kilometers of scattering to build up the blue you see overhead, which is also why distant mountains look bluish and hazy.
Something to Do Together
The Tyndall demonstration is still the best ten minutes of kitchen physics available. A tall clear glass of water, two or three drops of milk, and a flashlight in a dark room reproduce the sky (a bluish glow from the side) and the sunset (an orange beam viewed end-on), and a pair of polarized sunglasses rotated while viewing the side glow shows the polarization that bees use to navigate. If your child asks why milk works, the answer is that casein micelles in milk are small enough to scatter in the Rayleigh regime, at least until you add too much and the glass goes white.
Sources
- Strutt, J.W. (Lord Rayleigh). "On the light from the sky, its polarization and colour." Philosophical Magazine 41 (1871): 107–120, 274–279.
- Rayleigh, Lord. "On the transmission of light through an atmosphere containing small particles in suspension, and on the origin of the blue of the sky." Philosophical Magazine 47 (1899): 375–384.
- Tyndall, J. "On the blue colour of the sky, the polarization of skylight, and on the polarization of light by cloudy matter generally." Proceedings of the Royal Society of London 17 (1869): 223–233.
- Einstein, A. "Theorie der Opaleszenz von homogenen Flüssigkeiten und Flüssigkeitsgemischen in der Nähe des kritischen Zustandes." Annalen der Physik 33 (1910): 1275–1298.
- Smith, G.S. "Human color vision and the unsaturated blue color of the daytime sky." American Journal of Physics 73 (2005): 590–597.
- Bodhaine, B.A., Wood, N.B., Dutton, E.G. and Slusser, J.R. "On Rayleigh optical depth calculations." Journal of Atmospheric and Oceanic Technology 16 (1999): 1854–1861.
- Bohren, C.F. and Fraser, A.B. "Colors of the sky." The Physics Teacher 23 (1985): 267–272.
- Bohren, C.F. and Huffman, D.R. Absorption and Scattering of Light by Small Particles. Wiley (1983).
- NASA/JPL-Caltech. "Sunset on Mars" (Curiosity Mastcam, Gale Crater, April 15, 2015).