A frequently asked question by children is: “Why is the sky blue?” Some answer: “because it reflects the color of the ocean”, while others assume that this is because the oxygen is a blue-colored gas. However, if you do a quick search on the Internet, you will find that the sky is blue because of three simple factors: (1) sunlight is made out of light of many different wavelengths, (2) the Earth's atmosphere is made out of molecules that scatter different-wavelength light by different amounts, and (3) how our eyes perceive light.
As proved by Sir Isaac Newton with a triangular prism, when light passes through a prism; it breaks the white light into its constituent colors: red, orange, yellow, green, blue, indigo, and violet. Each color has a different wavelength as a unique fingerprint of its own, so when it passes through the prism, it travels at different speeds and refracts at different angles.
When the sunlight reaches the Earth’s atmosphere, it is scattered in all directions by all the gases and particles in the air. The white light from the Sun passes through the atmosphere; colors with longer wavelengths—such as red, orange, and yellow—pass through; while blue and violet wavelengths are absorbed by the gas molecules and are scattered across the sky. Your eyes see these reflected wavelengths as blue color because blue and violet are just the right wavelengths to bounce off the particles of gas in the atmosphere, so they get separated from the other colors making the sky appear blue to us.
Since scattering by the atmosphere causes the sky to be blue, a planet with no atmosphere cannot have a bright sky. The Moon’s atmosphere is so thin; when the air is too thin, gas molecules do not collide with each other. As a result, sunlight is not scattered and, whether it is daytime or nighttime on the Moon, the sky appears black. This was proven by the photographs taken by Apollo astronauts on the Moon, showing a completely dark sky in all directions during daylight.
Likewise, Mars has a very thin atmosphere mostly made of carbon dioxide and filled with fine dust particles; these fine particles scatter light differently than the gases and particles in Earth’s atmosphere. Photos from NASA’s rovers and landers on Mars have shown the opposite of what you would experience on Earth. During daytime, the Martian sky takes on an orange or reddish color; but, as the Sun sets, the sky around the Sun begins to take on a blue-gray tone.
The blue color of the sky is a result of a specific type of scattering known as Rayleigh scattering; this type of scattering depends on the wavelength of light and it scatters light off particles that are no bigger than one-tenth the wavelength of the light. In the lower atmosphere, tiny oxygen and nitrogen molecules scatter short-wavelength light—such blue and violet light—to a far greater degree than long-wavelength light—such as red and yellow.
If both blue and violet have short-wave lengths, why do we see the sky blue and not violet although the atmospheric particles scatter violet more than blue? The answer is simply because some of the violet light is absorbed in the upper atmosphere and our eyes are more sensitive to blue light than violet.
When the Sun is high in the sky, the sky appears a brighter blue because there is more atmosphere to see in those directions, and therefore, more blue light. During sunrise/sunset or moonrise/moonset, the light from the Sun, or the Moon, has to pass through tremendous amounts of atmosphere; the closer to the horizon it is, the more atmosphere the light must pass through. While the blue light gets scattered in all directions, the red light scatters much less efficiently. This means that both the light from the Sun’s disk, or Moon’s, turns to a reddish color, but also the light from the neighborhood of the Sun and Moon—the light that hits the atmosphere and scatters just once before reaching our eyes—is reddened at that time.
In addition to the scattering of light in the atmosphere, our eyes have a specific technique to perceive colors. The eyes have three types of cones for detecting colors, in addition to the monochromatic rods; when it comes to detecting a color, our brain receives signals from all four to be interpreted into a color. Each type of cone, plus the rods, are sensitive to the light of a different wavelength. Our eyes respond more strongly to blue, cyan, and green wavelengths of light than they do to violet; even if there is more violet light, it is not enough to overcome the strong blue signal our brains deliver.
The next time you are asked why the sky is blue, you will probably know what to say without hesitation. The sky is just our atmosphere as we see it from underneath, in addition to the sensitivity of our eyes to colors. if there was no scattering or absorption, the sky would appear black in the daytime and if there was more absorption or scattering, the sky might appear to be yellow, orange, or red all day long.
References
forbes.com
livescience.com
pectrumnews1.com
scientificamerican.com
spaceplace.nasa.gov
wonderopolis.org
This article was first published in print in SCIplanet, Spring 2020 Issue.
Cover image: Sea sky photo created by lifeforstock - www.freepik.com