Here in the northern hemisphere, the longest days of the year are approaching and with it come nature’s daily masterpiece: sunset.
Those colors we see during sunsets are actually the result of some light physics.
Light from the sun doesn’t have a color; rather, it has all of them, each with a specific wavelength that determines the color we see when the wavelength bounces back to our eyes. When we look to the sky, the colors we see depend on how the sun’s light travels through the air.
“The two main molecules in air, oxygen and nitrogen, are very small compared to the wavelengths of the incoming sunlight—about a thousand times smaller,” Stephen Corfidi, a meteorologist with the the US National Oceanic and Atmospheric Administration, tells National Geographic.
“That means that they preferentially scatter the shortest wavelengths, which are the blues and purples.”
The reason the sky doesn’t seem purple, he says, is because our eyes are best at picking up green light wavelengths, which are closer in length to blue. (Skies on other planets with different air molecules are different colors.)
During the day, sunlight travels to us from a shorter path through the atmosphere, so it doesn’t run into that many air molecules. At sunset, though, the sunlight we see has traveled a longer route, and most of the shorter wavelengths have been reflected away from our eyes. Reds, oranges, and yellows all have longer wavelengths, which is why they reach us, as Scientific American explains.
Pollutants in the air also color sunsets. Natural particulates from ocean spray, volcanic ash, or smoke from fires can all linger in the air and change the way sunlight is reflected. These tiny flecks are responsible for truly vibrant reds, for instance. Artificial contaminants, like smog, reflect the light differently, too.
“Large particles in the lower atmosphere tend to mute and muddy the colors because they absorb more light and scatter all the wavelengths more or less equally,” says Corfidi. “You don’t get that dramatic filtering effect.” They can still be beautiful, although too much smog can hide the sun entirely.
Some sunsets are so gorgeous you may be inspired to take a picture. When you do, remember that air, and the particles in it, are the ultimate filter.