How many times have you been asked by a child, or an adult for that matter, why the sky is blue? Probably more than you can count. If you type “why” into your Google search bar, it’s the fourth result to automatically fill-in your search. Let’s take some time to tackle this question, so next time you’re asked, you can answer with confidence.
White light is composed of a visible spectrum of seven colors: red, orange, yellow, green, blue, indigo, and violet (ROY G BIV, for those who prefer acronyms). Each of these colors has a different wavelength, with red being the longest and violet the shortest.
Light travels in straight lines until something reflects it (like a mirror), refracts it (like a prism), or scatters it. The gaseous molecules and particles of the atmosphere scatter the whole spectrum of the sun’s light. Due to Rayleigh scattering–the shorter the wavelength, the more it scatters–blue becomes the most diffused color throughout the atmosphere. At PSI, we use Rayleigh Scattering every day to measure the molar mass averages of polymers.
If violet has the shortest wavelength, then why isn’t the sky violet? First, not all white light is the same. The white light spectrum emitting from the sun is not constant for every wavelength. Sunlight emits much weaker violet beams than blue, which are more easily scattered to the point of invisibility in the upper atmosphere by nitrogen and oxygen. Second, human eyes are less sensitive to violet, making it far harder for us to see it.
So blue wavelengths are scattered again and again by the molecules in the atmosphere, turning our sky blue. When the sun sets, however, the light must pass through more of the atmosphere. Because more atmosphere and denser atmosphere (the atmosphere is denser closer to Earth’s surface) means more particles, blue light is scattered to the point of obscurity, while long wavelength colors (reds & oranges) scatter more. Thus, reds & oranges become visible.
That’s all well and good on the physical science side of things, but there’s more. What’s fascinating about all this is that, to some degree, the sky is blue because we say it is. Depending on the day, the weather, the sun’s position, and many other factors, the sky can be any number of colors other than blue. However, we’ve internalized “sky=blue”. Perception shapes cognition, but cognition can also affect perception.
Take, for example, the most recent color debate to tear apart the internet: tennis balls. This issue wasn’t as divisive as, say, the now-infamous blue/black (or gold/white) dress, but still led to many an office battle via email, Slack, or the water cooler. The question: is a tennis ball yellow or green?
Officially, it’s yellow. But, like with the dress-that-shall-not-be-named and the sky, its color is equal parts perception and cognition. Bevil Conway, a researcher of color perception in the human eye, explains that “like the color of a lot of objects, how we label [a tennis ball] is determined both by perceptual and cognitive factors: the actual physical light entering your eye and … knowledge about what people have typically labeled the objects.”
For The Dress, we didn’t know what color the dress should be, because there is no cognitively ingrained standard for dress color. Instead, it was completely controlled by our brains’ natural color correction process, which led to differences in perception: “Our visual system is supposed to throw away information about the illuminant and extract information about the actual reflectance,” according to Jay Neitz. Daylight changes colors, so in order for us to see the color of objects, our neural receptors must adjust for light difference. “What’s happening here is your visual system is looking at this thing, and you’re trying to discount the chromatic bias of the daylight axis,” Conway explains. So depending on whether you discount the gold side or the blue side determined how the dress appears. For The Great Tennis Ball Debate of 2018, it’s probable that something similar is happening, along with a lack of cognitive knowledge.
While there’s no debate over the color of the sky, the process of perceiving it as such is the same. The molecules in the sky are scatterers, meaning we discount the illuminant (sunlight) and see only the scattered blue wavelengths. And because we’ve been cognitively trained to see the sky as blue, it’s blue.
It’s always an interesting day when a simple question leads to a interdisciplinary answer. It’s a good reminder to us that learning requires expanding an idea beyond one field of study to see how our understandings of the world can work together.