Why is the sky blue?

Take a seat and squeeze in

The trip starts at our star, the Sun. Inside the Sun, as in any star, a series of chemical reactions take place producing large amounts of light. As you know, even if you can only briefly look at the Sun directly, the sunlight is white. You have probably heard that white light is composed of various colors: red, orange, yellow, green, blue, indigo, and purple. Therefore, imagine that on our trip the passengers are very tiny people of all these colors. In reality, instead of tiny people, the light is composed of waves of different lengths and our eyes interpret each one as a different color. You can imagine that the red passengers are slightly larger than the orange ones (because the wavelength of the red color is longer), the orange ones are larger than the yellow ones, and so on. You can also imagine them traveling very tightly packed, with hardly any space between them, as if the Sun-Earth trips were operated by Ryanair.

A calm flight…

On the way from the Sun to the Earth, there are no obstacles because, as you know, in space there is no air, it is a vacuum. Therefore, the light travels a straight path, without any disturbance. You can imagine that passengers have a pretty boring ride for the most part: no curves, no braking, nothing. It's not very exciting¹. Luckily, light travels very fast and after about 8 minutes of boring traveling, it is announced: "Dear passengers, we are approaching the last 100 km of travel: The Earth's Atmosphere. Please prepare for landing, - and believe it or not - make sure your belt is properly unfastened". Yes, it is not a typo. Once the light reaches the Earth's atmosphere, the passengers will no longer be packed.

… and a turbulent landing

Things are different in the atmosphere. After traveling through an unaltered vacuum passengers now face a new environment full of obstacles. The Earth's atmosphere is composed mainly of nitrogen (the most abundant), oxygen and others such as carbon dioxide. Apart from these gases, there is a lot of dirt: dust, pollen, sand, pollution... The molecules of these gases together with the particles suspended in the air will represent real obstacles for some of our passengers. Larger passengers, such as reds, oranges or yellows, have no big issue. Since they are larger than most obstacles in the air, their trajectory is not heavily altered and they can easily reach the end of the trip, the Earth's surface. However, it turns out that these obstacles are similar in size to smaller passengers, such as the blue ones. After colliding with the obstacles, the smaller passengers are scattered in a completely different direction. Then, they collide with another obstacle, they are scattered, and so on. Scientists call this Rayleigh scattering. As you may have noticed, from the initial group of passengers, only the largest passengers (red, orange and yellow) reach our eyes, while the smaller ones (blue, indigo and violet) are trapped in the labyrinth of obstacles in the air staining the sky with the characteristic bluish color.

Other colors

You might wonder why the sky is not violet. Well, the fact is that it is. Violet passengers are scattered in the air just as much as blue ones. But our eyes have a higher sensitivity to blue than to violet. Our retina has sensors (called cones) that are sensitive to red, green and blue. Just with these three types of sensors, our brain can mix these three colors to obtain any other color. The brain does something similar to what you used to do at primary school mixing paint of different colors, except with just red, green and blue. For example, if in a part of your retina the cones sensitive to the colors red and green are excited simultaneously, your brain will interpret it as the yellow color. As you can see, it is less difficult for our brain to detect blue because we have cones that are specialized in detecting it. However, to detect violet, our brain has to combine shades of red and blue, which is tedious.

And why does the sky turn orange at sunrise and sunset? Well, our privileged passengers, the largest ones, are less lucky when the Sun's rays do not fall perpendicular to the surface of the Earth. At dawn, the Sun's rays form a very small angle with the Earth's surface, which means that the passengers that reach our eyes have traveled larger distances within the Earth's atmosphere. Although the air obstacles do not substantially deviate the large passengers, they do deviate them to some extent. The more particles you put in their path, the more you will scatter them. At sunrise and sunset, the distance that the Sun's rays have to travel to reach you is so large that the number of obstacles they collide with is high enough to also scatter the characteristic red, orange and yellow colors.

In fact, you have seen light scattering in other occasions. Something very similar happens with rainbows. In these cases, the light collides with water droplets that are much larger than other particles in the air and deflect all colors equally, slightly separating passengers from their seats (as if after going through a water droplet they were automatically upgraded to business class). This separation is just enough so that we can enjoy each passenger individually in a beautiful rainbow. You have also seen it when it is about to rain. Clouds turn dark because they are so dense that the water droplets do not even let light through, no matter the color. Thanks to this you know when to take out your umbrella.

A fragile shield

To conclude, I want to emphasize again that our atmosphere is only 100 km above the surface². This strikes me because the radius of the Earth is more than 6000 km long. The atmosphere provides us with the oxygen we breathe and protects us from stuff that would kill us, among others, from ultraviolet radiation and meteorites. To give you an idea, if the Earth were a desktop globe, the atmosphere would be nothing but the sticker that shows the illustrations of the countries. If the Earth were the apple from CEBE's logo, the atmosphere would be no thicker than the skin. As you can see, it is an extremely thin and fragile layer but it allows human beings to live on this planet. I cannot think of a higher priority for humankind than making sure this beautiful blue roof remains.

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Note 1: That a photon would be very bored is not entirely true. Not only because a photon has no feelings, but also because the theory of relativity tells us that for a photon traveling at the speed of light in a vacuum, time does not pass! Therefore, although for us it takes about 8 minutes for light to get from the Sun to the Earth, for a photon this happens immediately. But let us save this for another post.

Note 2: 100 km is the formal limit that is used to separate the atmosphere from outer space. Interestingly, if you could go up to this height you would no longer see the sky blue since at such a height there are no gases or dust particles – and thus, no Rayleigh scattering. Also, you could see the stars even in bright sunlight. The reason why we do not see the stars during the day is that they are hidden by the scattered blue light.