Answer: Oxygen and nitrogen molecules in the atmosphere scatter sunlight incoming, with blue wavelengths being scattered more often than red ones resulting in the sky appearing bluer than usual.
But this explanation doesn’t explain why the sky doesn’t turn violet at sunset or why Martian skies have an earth-tone hue; that is because sunlight must travel further through thicker atmospheres before reaching us.
The Sun
As sunlight passes through our atmosphere, it reaches our eyes in the form of multiple colors that form white light. As light passes through our atmosphere and interacts with air molecules, its wavelengths get scattered in all directions; blue wavelengths disperse less than red ones which results in our perception that sky appears bluer.
As is visible even on other planets with thicker atmospheres than Earth, sunlight still plays an integral part in creating our skies’ colors. Not only is the Sun a massive source of energy and radiation for life on Earth; but its colors also play an essential part in making life possible here on our home planet.
The Sun is an immense, bright star that emits electromagnetic waves in the form of light and heat, reaching Earth’s atmosphere where they’re scattered by atmospheric particles and water vapor, but shorter wavelengths such as blue and violet pass through without interference from red and yellow wavelengths, making most incoming sunlight look blue when looked directly upon by our planet’s star.
As the sun nears its horizon at sunrise and sunset, its light must travel further through our atmosphere, forcing its rays to strike at more oblique angles with more blue and violet wavelengths scattered. As a result, more red and yellow wavelengths reach our eyes instead; creating an atmospheric “rosy glow.”
No single factor explains why our skies are always blue, but one key one is certainly the Sun. Without its powerful light and energy output, our skies would likely appear dull gray. But thanks to the Sun, our atmosphere – composed of nitrogen, oxygen, water vapor, chemical pollutants, dust particles and ozone – is better at reflecting blue wavelengths of sunlight than other colors.
The Earth’s Atmosphere
As sunlight enters Earth’s atmosphere, it is redirected and absorbed by various gases, producing indirect sunlight that forms most of what we perceive, giving the sky its blue hue. The reason for this phenomenon is simple – oxygen and nitrogen molecules scatter blue wavelengths more readily than longer red or longer wavelengths; you can test this theory for yourself by shining a light through water containing milk; you will notice deeper blue hues from it than through clear water.
This effect is more intense near the horizon because light has traveled further through the atmosphere and contains more dust and pollution particles larger than wavelengths of light, which scatter reds and yellows more than blues (Rayleigh scattering). When Sunrays hit lower angle at sunset and sunrise, more blue wavelengths are scattered due to Rayleigh scattering than when approaching directly through upper atmosphere; consequently, visible spectrum reaching your eyes becomes a mix of blues and yellows than during midday hours.
Blue hues scatter more efficiently than other colors because their shorter wavelengths create a stronger electromagnetic wave that reaches your eyes – this is because your eyes are more responsive to intensity of these waves rather than wavelength.
So if your skies are filled with clouds or dense haze that blocks sunlight from reaching us, they may appear more of a pure blue than usual. Furthermore, as midday sun passes through more layers of atmosphere to reach us and scatters blue and violet wavelengths even more than normal, the color becomes increasingly scattered.
There are multiple layers in our atmosphere, each differing in temperature and gas composition with increasing altitude. Atmospheric layers vary based on proximity to Earth’s surface: troposphere contains most clouds nearer the ground and nearly all weather occurs here; stratosphere contains jet streams as well as the ozone layer that gives sunset sky its distinctive red hue; mesosphere covers an area up to 100 kilometers before finally reaching interplanetary space.
The Human Eye
Light traveling through the atmosphere encounters gases and particles in the air that scatter it, causing some wavelengths to be absorbed while others pass straight through. Shorter wavelengths (like blue) tend to get scattered more than longer ones like red; this causes the sky to appear bluer.
Tyndall first observed and Rayleigh later explained this phenomenon of light scattering through Earth’s atmosphere, as various wavelengths are absorbed or scattered differently by gases and particles in various ways – giving rise to what is known as multiple reflections; which gives our sky its distinctive blue hue.
Smaller molecules found in the atmosphere such as oxygen and nitrogen tend to scatter blue light more than red, as their distance from blue wavelengths of light is closer than their distance from red wavelengths. On the contrary, larger molecules like those found in milk or water have more absorption capacity when it comes to reflecting red wavelengths back.
So the reds in sunlight are filtered out, leaving only blues and violets through to reach our eyes – explaining why the sky appears blue during midday, while red and yellow hues appear at sunset/rise times.
Human eyes are living optical devices and this helps explain why the sky always appears blue. The cornea and pigment layer in the sclera keep our eyes “light tight”, except for one small region in front of them called the pupil – a spherical lens through which light from above travels and gives off a hint of blue color as it passes through it.
The pupil also responds more strongly to certain wavelengths of light than others. Red cones are stimulated more strongly when exposed to scattered red wavelengths than orange and yellow ones, while blue and violet wavelengths respond strongly when subjected to blue-violet light rays – this phenomenon gives sky its characteristic hue.
Other Planets
A planet’s sky can vary depending on its composition and how light is absorbed or scattered through its atmosphere, though generally speaking if its atmosphere is transparent to sunlight it will appear blue to observers from outside its orbit.
Martian air may seem clear to our eyes, yet has an eye-catching yellow tint due to dust particles much larger than gas molecules using Mie scattering to absorb short wavelengths of blue light while scattering longer wavelengths such as red, orange and yellow rays for Mie absorption and Mie scattering – thus giving its sky its characteristic hue.
Nitrogen, which comprises most of the Earth’s atmosphere, scatters light preferentially in the blue spectrum. When the Sun rises and sets, Earth appears bluer due to an increase in absorption and scattering of shorter wavelengths that make up its electromagnetic spectrum’s blue section.
When we consider other planets in our Solar System, many appear to share similar atmospheric composition and chemical makeup to Earth – producing blue skies as seen here on our own planet.
However, when looking at planets from other star systems, their sky colors differ greatly. Jupiter and Saturn both boast blue skies while Uranus has an unusual cyan hue due to methane present in its atmosphere.
Planets with atmospheres composed of different gases may still have blue skies due to Rayleigh scattering giving their atmospheres their distinctive hue. Each atmosphere differs based on which molecules make up its atmosphere and how these behave in this regard.
Tyndall effect can also account for natural blue hues found in gem stones and the feathers of blue jays, or its sheen on freshly washed windows or ornamental glass ornaments; and is even used to manufacture blue glass for windows and light fixtures!