By Eleftheria Safarika
“Why is the sky blue?” It’s a question we all have asked when we were young, to which seldom we got a satisfying answer, apart from “Because it is”. Now, it’s time to really understand the scientific explanation of our ever unanswered question.
Let’s start from the basics. As you probably know, the white light that comes from the sun is actually a combination of light of all frequencies, a mixture of light of different wavelengths.
As the light travels through the atmosphere, it interacts with the particles of gas and dust existing there, “hitting” them. This is what the British scientist Lord Rayleigh found out, and is called Rayleigh scattering. Light of longer wavelengths, such as red light, scatters less while it travels through the atmosphere, while light of shorter wavelengths such as blue, scatters more. This is because red light, which is of a lower frequency (and thus of a longer wavelength), is able to pass through the particles and dust present in the atmosphere easily, without interacting, “hitting” them as much as blue light. Rayleigh found a formula to calculate this interaction, the scattering: I , meaning that the scattering’s intensity (I) is proportional to one over the light’s wavelength (λ), raised to the fourth power. So, light with a longer wavelength will make the denominator of this fraction larger, thus making the fraction itself smaller. Therefore large wavelengths have a small intensity of scattering. The opposite goes for shorter wavelengths of light.
However, a rational question that might arise to one’s mind would be: “Since violet light has an even shorter wavelength than blue, then why isn’t the sky violet?” There are many reasons the sky doesn’t appear violet. Firstly, the sun emits more light with the frequency of the blue light, than light with the frequency of the violet light, therefore blue is more dominant. Secondly, the violet we refer to as violet is not really a purple-ish shade, but rather a deeper blue. There is no single frequency matching to pure purple. Purple light is a combination of other single-frequency light.
We can understand this using this chromaticity diagram: The colors of the rainbow are the colors on the outer rim of the shape. Those within the whole internal area are created by combinations of the colors on either side of the shape. We can distinctly see that purple and all its hues are on the inside, created by combinations of red and blue.
Lastly, our eyes are designed in a specific way to perceive light. There are cones within them, meaning color-sensitive cells, which are responsible for this function. There are red, green, and blue cones.
Each category of cones is mostly responsible for perceiving light of a specific color, but is also very slightly sensitive to other frequencies of light as well. Two kinds of cones, also, could have an overlapping range of the colors they are able to perceive. For example, green light could be detected mainly with the green cones, but also slightly with the blue cones as well. The blue cones, now, are mainly responsible for detecting blue light, and less for violet. So we are more sensitive to blue light, therefore the blue-violet part of the spectrum the blue cones detect, which is the one that dominates in the sky, after the scattering, is mostly perceived as blue.
So now that we’ve understood why the sky appears blue during the day, we should go a bit forward. Why does it appear red during the sunset? This is mostly because during sunset, the sun has moved to such a position, where light rays have to travel more through the atmosphere to reach the earth. Therefore, blue light is scattered much more than when the sun is directly over the earth at noon, and thus takes much more time to reach our eyes, since it is “too busy” being scattered. Red light, which, as we said, scatters less, reaches us first. This is why the sky takes at sunset a reddish/orange hue.
So, the blue light scatters much more, and takes more time to complete its journey, while lower-frequency light, such as red, can travel more easily, and reach our eyes first. Something else now, why do clouds seem white? Clouds are made up of water droplets, which are much larger than the molecules and dust in the atmosphere, and are large enough to scatter about all frequencies of light equally, as light enters through them. This kind of scattering is called Mie scattering. So, since all wavelengths are scattered, the light appears to be white, the combination of all wavelengths, making therefore the clouds seem white. If the clouds have a very large concentration of water droplets, or are in the shadow of other clouds, then light cannot pass easily through them, and they appear gray.
It is worth mentioning here that mainly responsible for all these phenomena is the earth’s atmosphere. It is what, apart from all its other functions which support life, causes light to scatter, and makes the sky appear as it does. If we looked at the sky from the surface of the moon, however, we would not see it blue, but black. This is because the moon does not have an atmosphere to scatter the sunlight.
All the mentioned factors have managed to create the beautiful sky we see from our position on the surface of the earth, and enjoy every day. It is truly amazing to see how science is able to answer questions ranging from simple everyday speculations, such as why the sky is blue, to serious scientific problems, regarding topics even such as black holes and quantum mechanics. But the charm to be able to explain everyday life is unique and irreplaceable.
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