I’ve been writing more about light recently, so I wanted to cover a basic question that most people first ask as children: why is the sky blue? We can tell that the blue color of the sky is related to sunlight, because at night, we can see out to the black of space and the stars. We also know it’s related to the atmosphere, because in photos from places like the Moon which have no atmosphere, the sky is black even when the Sun is up. So what’s going on?
When light from the Sun reaches Earth, its photons have a combination of wavelengths (or energies), and we call the sum of all of those the solar spectrum. Some of these wavelengths of light are absorbed by particles in the atmosphere, but others are scattered, which means that the photons in question are deflected to a new direction. Scattering of light happens all around us, because the electromagnetic wave nature of photons makes them very sensitive to variations in the medium through which they travel. Other than absorption of light, scattering is the main phenomenon that affects color.
There are a few different types of scattering. We talked about one type recently, when discussing metamaterials and structural color: light can be scattered by objects that are a size similar to the wavelength of that light. That is called Mie scattering, and it’s why clouds appear solid even though they are mostly empty. The clouds are formed of tiny droplets, around the size of the visible wavelengths of light, and when these droplets scatter white light, the clouds themselves appear diffuse and white. Milk also appears white because it has proteins and fat in tiny droplets, suspended in water, which scatter white light.
However, even objects much smaller than the wavelength of light can induce scattering. The oxygen and nitrogen molecules in the atmosphere can also act as scatterers, in what’s called Rayleigh scattering (or sometimes the Tyndall effect). For Rayleigh scattering, these molecules can be affected by the electromagnetic field that the photon carries. A molecule can be polarized, meaning the positive and negative charges in the molecule move in opposite directions, and then the polarized molecule interacts with the light by scattering it. But, the polarizability of individual molecules depends on the wavelength of the incoming light, meaning that some wavelengths will scatter more strongly than others. When Rayleigh worked out the mathematical form of this dependence, in 1871, he found that the scattering was inversely proportional to the fourth power of the wavelength of light, which means that blue light (which has a smaller wavelength) will scatter much more strongly than red light (which has a larger wavelength).
Thus, we see the Sun as somewhat yellow, because only the longer wavelength light in red and yellow travels directly to us. The shorter wavelength blue light is scattered away into the sky, and comes to our eyes on a very circuitous and scattered route that makes it look like the blue light is coming from the sky itself. At sunset, the sun appears even redder because of the increased amount of atmosphere that the light has travelled through, scattering away even more blue light. And, when there is pollution in the air, the sun can appear redder because there are more scattering centers that scatter away the blue light.
Of course, the fact that blue light scatters more is only half the story. If that were all there is to it, we’d see the sky as a deep violet, because that’s the shortest wavelength of light that our eyes can see. But even though we can see the violet in a rainbow, our eyes are actually much less sensitive to it than they are to blue light. Our eyes perceive color using special neurons called cones, and of the three types of cones, only one can detect blue and violet light. But the blue cone’s response to light peaks at around 450 nm, which is right in the middle of the blue part of the spectrum. So we see the sky as blue because it is the shortest wavelength that we’re capable of detecting in bulk. Different particles in the air can change the color of the sky, but so would different ways of sensing color. So Rayleigh scattering determines which light is scattered, and our visual system determines which of that light we see best: sky blue.