Color in Nature

Sunlight arriving at the top of the atmosphere is essentially white - a full spectrum. Every color we see in the sky comes from the atmosphere sorting that light: scattering bounces some wavelengths in new directions, absorption removes a little, and refraction bends and separates it. The colors are not added by the air; they are redistributed from the same white sunlight, differently depending on the geometry and what the light passes through.

Two kinds of scattering do most of the work. Rayleigh scattering by tiny air molecules is strongly wavelength-dependent and paints the blue sky and red sunsets. Mie scattering by larger particles - droplets, dust - is nearly wavelength-neutral and makes clouds and haze white. Get those two ideas and most of the sky makes sense.

Rayleigh scattering gets dramatically stronger toward the blue end: its strength rises as roughly one over the wavelength to the fourth power. Violet and blue (short wavelengths) scatter many times more than red (long). So in every direction of the daytime sky you see scattered blue. As the sun sinks, its light takes a far longer path through the air - more airmass - and along that path almost all the blue is scattered out before it reaches you, leaving the red and orange that survive. Drag the sun down and watch it happen.

A rainbow is sunlight refracted, reflected, and refracted again inside raindrops. Each drop bends the different wavelengths by slightly different amounts (dispersion), and the geometry concentrates the light back toward you at a fixed angle: the primary rainbow at about 42° from the antisolar point (the shadow of your head), red on the outside, violet on the inside. A second, fainter secondary rainbow at about 51° comes from a double internal reflection - and its colors are reversed. Between them lies the darker Alexander's band.

If scattering makes the sky blue, why are clouds white? Because cloud droplets are thousands of times larger than air molecules. At that size, scattering switches from Rayleigh's strong color preference to Mie scattering, which is nearly the same for all wavelengths. Scatter every color equally and the result is white. The same rule explains fog, milk, salt, and snow. Grow the particle size and watch blue fade to white.

Once you know scattering and refraction, a whole catalog of sky phenomena falls out of the same rules. Pick one to see why it looks the way it does.

Not all of nature's color is in the sky, and not all of it is pigment. Many of the most vivid natural colors are structural - made by microscopic architecture that interferes with light, the subject of the colorants article. A blue jay's feather has no blue pigment; tiny structures scatter blue back. A morpho butterfly and a peacock shimmer because layered nanostructures reinforce specific wavelengths. Even some blue eyes are structural, not pigmented - the same Rayleigh-like scattering that blues the sky, working in the iris.

Six questions on scattering, sunsets, rainbows, and clouds. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.

Natural color is applied physics of light. The numbers reflect each article's position in the editorial roadmap.