Sunlight is blue because of what’s known as Rayleigh scattering. When sunlight passes through Earth’s atmosphere, it gets scattered by all the gases and particles in it. Light at shorter wavelengths (blue) is scattered more than other colors of light.
Some people say that the ocean makes the sky blue because it reflects the color of the sky, but this isn’t true. The real reason is that the ocean absorbs red and orange wavelengths more than blue.
Light
There are a lot of things that can affect how the sky appears, such as smog, dust and water vapor. But the blue color we see in most cases is a result of light scattering off gas molecules. When white light from the Sun hits a gas molecule, it gets slowed down and its wavelengths get shifted. When this happens, shorter wavelengths like blue are more easily scattered than longer wavelengths like red. This is a process called Rayleigh Scattering and it was named after John William Strutt, 3rd Baron Rayleigh, who developed a model of this effect in 1871.
In the case of the sky, when a beam of white sunlight hits the atmosphere, it gets slowed down by the oxygen and nitrogen molecules in the air. Then its wavelengths are shifted, and the blue ones are more easily scattered than reds and oranges. That’s what gives the sky its distinctive blue hue.
It’s also why open water looks blue — again, it has to do with the way different wavelengths of sunlight interact with the surface. And it’s why sunsets and sunrises appear to be red — again, because as the Sun gets lower in the sky, more of its blue light is absorbed by the atmosphere than the orange and red light. This is also the reason why urban areas seem to glow a little at night, as the blue light from streetlights is more readily scattered than the other colors.
Oxygen
Oxygen (O) is an odourless, colourless gas in the Earth’s atmosphere. It is a member of the non-metals family and is found in all living organisms as well as in the oceans and sand. It is the most abundant element on Earth. Normally it exists as diatomic molecules, O2, but can also exist in a triatomic form, O3, known as ozone. It is paramagnetic and attracts electrons to itself, which makes it very useful in combustion processes.
If oxygen disappeared from the Earth’s atmosphere the air would become a much denser gas, with less ability to scatter blue light. This would make the daytime sky a much darker colour, while at night it would be completely black. Airplanes and birds would be unable to fly, and people at the beach would get sunburns as there would be no ozone to absorb UV radiation.
In open water the colour of the sea or ocean is due to a combination of things, including how sunlight interacts with different substances and the fact that some wavelengths of light are more strongly absorbed than others. However, it is not the case that the colour of the sky is reflected in the water; the water’s blue appearance is because it is naturally so.
Water
The sky looks blue because gas molecules in the atmosphere scatter and absorb light, causing the shorter wavelengths of blue and violet to be scattered more than red. When these reflected wavelengths reach your eyes, your brain interprets them as blue.
Water molecules do not act the same way as gas molecules, however. When sunlight hits them, they preferentially absorb the red and orange wavelengths of light. This explains why open water appears to be blue in color.
As you go higher in the atmosphere, the amount of water that scatters the blue wavelengths decreases. This means the horizon fades into a paler shade of blue. The reason for this is that to reach you, the blue light must travel further through the atmosphere. As it does, the light is rescattered in other directions and fewer of the blue wavelengths make it to your eye.
In addition, the presence of clouds can affect how the sky looks. Dust and pollution in the air can give it a brown, yellow or red hue. In contrast, a clear daytime sky can be blue. The same thing explains why the Martian sky appears red when it’s cloudless, while the Moon lacks an atmosphere and is thus black. It’s also why, even on the clearest of days, the Sun appears red during a sunset or sunrise.
Rayleigh Scattering
As the Sun moves through the sky, its light is scattered off the neutral atoms of oxygen and nitrogen in the atmosphere. This is called Rayleigh scattering, and it gives the sky its blue color. The shorter wavelengths of blue and violet are scattered much more strongly than the longer wavelengths of red and yellow. This is because the gas molecules have different masses and hence vibrate at a lower frequency for these colours.
Early physicists such as John Tyndall and Lord Rayleigh thought the blueness of the sky was due to dust particles and droplets of water vapour in the air, but later realised that this was not correct. They were right, however, that the atmosphere scatters blue light more than other colours. Rayleigh scattering is a relatively narrow form of optical scattering, and the amount of scattering is proportional to the frequency of the light to the fourth power.
This means that the higher frequencies of the spectrum (i.e., the shorter wavelengths) are scattered more strongly than the lower frequencies (i.e., the longer wavelengths). This effect makes a clear daytime sky look blue, but it doesn’t explain why it isn’t purple. The explanation for this lies in the absorption of a small fraction of light by hydroxyl and chlorine ions in the atmosphere, as well as other colour-causing molecules like aerosols and dust.
