If you’ve ever been curious about the world you live in, you’ve probably wondered why the sky is blue. The incorrect answers that people often give in response include:
that sunlight has a blue tint,
that oxygen itself is a blue-colored gas,
or that the sky reflects the oceans.
While none of those answers are correct, that last attempt brings up a related question that people often wonder about: why are the oceans blue?
As seen from space, planet Earth is often described as a pale blue dot, but it’s only the liquid bodies of water — dominated by Earth’s oceans — that appear blue-hued. The continents, clouds, and ice caps don’t appear blue at all; it’s the oceans, not the atmosphere, that give our planet its overall complexion. For thousands of years, humanity had to simply accept these properties of our world as facts. But with the advances of modern science, we understand why both the skies and oceans are blue.
When the Sun is high overhead, the sky towards the zenith is a much darker blue, while the sky towards the horizon is a lighter, brighter cyan color. This is due to the larger amount of atmosphere, and the larger amount of scattered light, that is visible at low angles on the sky. (Credit: pxfuel)
Contrary to what you might have read, there’s no one single factor responsible for Earth’s blue skies.
The skies aren’t blue because sunlight has a blue tint; our Sun emits light of many different wavelengths, and that light sums up to be a net white color.
Oxygen itself isn’t a blue-colored gas, but rather is transparent to light.
However, there are a myriad of molecules and larger particles in our atmosphere that do play a role, scattering light of different wavelengths by different amounts. The ocean plays no role in the color of the skies, but the sensitivity of our eyes absolutely does: we do not see reality as it is, but rather as our senses perceive it and our brain interprets it.
These three factors — the Sun’s light, the scattering effects of Earth’s atmosphere, and the response of the human eye — are what combine to give the sky its blue appearance.
Schematic animation of a continuous beam of light being dispersed by a prism. If you had ultraviolet and infrared eyes, you’d be able to see that ultraviolet light bends even more than the violet/blue light, while the infrared light would remain less bent than the red light does. (Credit: Lucas Vieira/Wikimedia Commons)
When we pass sunlight through a prism, we can see how it splits up into its individual components. The highest energy light is also the shortest-wavelength (and high-frequency) light, while the lower energy light has longer-wavelengths (and low-frequencies) than its high-energy counterparts. The reason light splits up at all is because wavelength is the critical property that determines how light interacts with matter.
The large holes in your microwave allow short-wavelength visible light in-and-out, but keep longer-wavelength microwave light in, reflecting it. The thin coatings on your sunglasses reflect ultraviolet, violet, and blue light, but allow the longer-wavelength greens, yellows, oranges, and reds to pass through. And the tiny, invisible particles that make up our atmosphere — molecules like nitrogen, oxygen, water, carbon dioxide, as well as argon atoms — scatter light of all wavelengths, but preferentially are more efficient at scattering bluer, shorter-wavelength light.. . . . . . . .
Rayleigh scattering affects blue light more severely than red, but of the visible wavelengths, violet light is scattered the most. It’s only due to the sensitivity of our eyes that the sky appears blue and not violet. The longest-wavelength and shortest-wavelength visible lights experience a difference in Rayleigh scattering by nearly a full order of magnitude. (Credit: Robert A. Rohde/Wikimedia Commons)
There’s a physical reason behind this: all the molecules making up our atmosphere are smaller in size than the various wavelengths of light that the human eye can see. The wavelengths that are closer to the sizes of the molecules present will scatter more efficiently; quantitatively, the law it obeys is known as Rayleigh scattering.
The violet light at the short-wavelength limit of what we can see scatters over nine times more frequently than the red, long-wavelength light at the other end of our vision. This is why, during sunrises, sunsets, and lunar eclipses, red light can still pass efficiently through the atmosphere, but the bluer wavelengths of light are practically non-existent, having been preferentially scattered away.
Some opalescent materials, like the one shown here, have similar Rayleigh scattering properties to the atmosphere. With white light illuminating this stone from the upper right, the stone itself scatters blue light, but allows the orange/red light to preferentially pass through undeterred. (Credit: optick/flickr)
Since the bluer wavelengths of light are easier to scatter, any incoming direct sunlight will become redder and redder the more atmosphere it passes through. The remainder of the sky, however, will be illuminated by indirect sunlight: light that strikes the atmosphere and then gets redirected towards your eyes. The overwhelming majority of that light will be blue in wavelength, which is why the sky is blue during the day.
It will only take on a redder hue if there’s enough atmosphere to scatter that blue light away before it reaches your eyes. If the Sun is below the horizon, all the light has to pass through large amounts of atmosphere. The bluer light gets scattered away, in all directions, while the redder light is far less likely to get scattered, meaning it takes a more direct path to your eyes. If you’re ever up in an airplane after sunset or before sunrise, you can get a spectacular view of this effect.
From very high altitudes in the pre-sunrise or post-sunset skies, a spectrum of colors can be seen, caused by the scattering of sunlight, multiple times, by the atmosphere. Direct light, from close to the horizon, reddens tremendously, while far away from the Sun, indirect light only appears blue. (Credit: Rnbc/Wikimedia Commons)
This might explain why sunsets, sunrises, and lunar eclipses are red, but might leave you wondering why the sky appears blue instead of violet. Indeed, there actually is a greater amount of violet light coming from the atmosphere than blue light, but there’s also a mix of the other colors as well. Because your eyes have three types of cones (for detecting color) in them, along with the monochromatic rods, it’s the signals from all four that need to get interpreted by your brain when it comes to assigning a color.
Each type of cone, plus the rods, are sensitive to light of different wavelengths, but all of them get stimulated to some degree by the sky. Our eyes respond more strongly to blue, cyan, and green wavelengths of light than they do to violet. Even though there’s more violet light, it isn’t enough to overcome the strong blue signal our brains deliver, and that’s why the sky appears blue to our eyes.
