Why is the sky blue?

Jorge Barrasa Fano · 26-04-2021 10:00 · CEBE answers

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.


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.


Jorge, me ha gustado muchísimo tu artículo. Increíblemente claro y con un símil muy acertado.

Angel Fuentes , 26 Apr 2021

Me ha encantado! Fotones que viajan en Ryanair jaja nunca lo olvidaré! ????

Ana Barragán , 26 Apr 2021

Perfectamente explicado. Muchas gracias!

Begoña Fano , 26 Apr 2021

Nice and memorable, thank you

Arne , 26 Apr 2021

Genial. Muy clarito y muy didáctico. Gran divulgador científico. Bravo!!!!
Siguiendo con los colores se me ocurre que podrías escribir otro post explicando por qué las plantas son verdes y no violetas o rojas. ¿Qué te parece?

José María Barrasa , 26 Apr 2021

Que fantasía! me ha encantado ^^

Elena , 28 Apr 2021

Estupenda explicación, interesante y didáctica. Un claro ejemplo de que la ciencia puede llegar a todos los públicos de una forma sencilla y amena. Enhorabuena por esa facilidad divulgativa que demuestras.

Francisco DAT , 03 May 2021

Qué bien explicado, divulgador de nacimiento!

Amalia , 20 May 2021

<a href=https://proxyspace.seo-hunter.com/mobile-proxies/karachev/>proxy bajo bots</a>

Norbethoora , 31 Jan 2022

"Мисс Марвел" [2022 - <a href="http://bit.ly/ms-marvel-1-8-seriya">Мисс Марвел (сериал). Смотреть онлайн</a> Сериал Мисс Марвел 1 сезон 1,2 серия LostFilm смотреть

ms-marvelxyeaq , 25 May 2022

Сериал Мисс Марвел 1 сезон смотреть онлайн все серии подряд в хорошем качестве hd 1080 <a href="http://bit.ly/ms-marvel-1-8-seriya">Мисс Марвел 1 сезон. смотреть онлайн</a> Сериал Мисс Марвел 1 сезон смотреть онлайн все серии подряд в хорошем качестве hd 1080

ms-marvelubixh , 26 May 2022

Один из конкурсов у команд снять <a href="https://bit.ly/FILMFILMFILM">фильм</a>, короткий и смешной.

ms-marvelnfnim , 29 May 2022

<a href="https://bit.ly/FILMFILMFILM">фильм</a>», «Самый смешной <a href="https://bit.ly/FILMFILMFILM">фильм</a>», «Самый креативный <a href="https://bit.ly/FILMFILMFILM">фильм</a>», «Приз зрительских симпатий» и «Приз журнала

ms-marvelgtewy , 29 May 2022

Смотреть онлайн фильм. Смотреть онлайн. Фильм <a href="http://bit.ly/2GbFj2S">Голодные игры И вспыхнет пламя просмотр фильма онлайн</a> Смотреть порно бесплатно категории онлайн шд:

35363415 65093442 934852029098 18284485120032642172

62023690 43096155 229944124725 4364472796032957802

9248658 67529243 17294668473 83041536619714492794

ms-marvelanbqy , 03 Jun 2022

Где в своем городе можно посмотреть фильмы <a href="http://bit.ly/1-1-a">Фантастические твари и где они обитают фильм смотреть онлайн</a> Смотреть фильмы онлайн. Вся информация о фильмах и сериалах.

93494966 2523151 775267758061 50811265449385986170

86001123 88312555 16666355245 39825338561531452

78099342 71553823 542784123383 6079808796939034704

ms-marvelczvtu , 03 Jun 2022

Лучшие фильмы и сериалы: Перевод На русском языке. <a href="http://bit.ly/nasledie-lji-smotret-onlajn">Наследие лжи фильм смотреть онлайн</a> Tube — Смотрите фильмы онлайн бесплатно.

74965148 1737127 16942041242 79837493692199389605

59419526 22188837 908695857554 2810517128696841376

81645238 84389281 216271451763 49043031855939961326

ms-marveldcyjm , 04 Jun 2022

Смотреть лучшие фильмы онлайн в хорошем HD качестве. <a href="http://bit.ly/3aXZvix">Никто не сравнится с тобой смотреть фильм</a> Фильм в хорошем качестве hd 720 1080

70532398 55773390 331623859897 75026387314682773501

46625654 14181212 71936344007 11715660771725648193

70033130 85867253 615427528458 65938103620695908212

ms-marvelxocyy , 04 Jun 2022


25002457 19821498 717625162485 563191162048756139

7858869 40089871 972021242002 95265330813625116420

95261692 39036637 275284307303 10111256220051041059

lhtbjklhjqic , 13 Jun 2022


ldtwwtlgtsck , 16 Jun 2022


lscwwulbaeyf , 16 Jun 2022


leqhjelqvjin , 17 Jun 2022


letmmqlsphly , 17 Jun 2022


ladjjkllpntq , 17 Jun 2022


llmmnsljiezc , 18 Jun 2022


ljugemllsyft , 18 Jun 2022


lsfeaelkwzuk , 18 Jun 2022


ltieuvldudhv , 18 Jun 2022

free gay sex chat fcn
miai chat gay
<a href="https://free-gay-sex-chat.com/">free gay sex chat </a>

DeeynKlmr , 15 Sep 2022

gay chat las vegas
<a href=https://chatcongays.com>in gay chat what is an otter?</a>
gay chat random alternative

GennieKlmr , 16 Sep 2022

law school essay editing service
<a href=https://au-bestessays.org>best essay writing company</a>
help on writing an essay

MarrisKlmr , 20 Sep 2022

custom essay writing online
<a href=https://bestcampusessays.com>buy cheap essay</a>
write my essay affordable

DorolisaKlmr , 21 Sep 2022

write my essay affordable
<a href=https://besteasyessays.org>can i get someone to write my essay</a>
academic essay writing help

MartyKlmr , 22 Sep 2022

buy an essay paper
<a href=https://bestessayreviews.net>assignment essay help</a>
college essay editing service

MerolaKlmr , 22 Sep 2022

national junior honor society essay help
<a href=https://bestessaysden.com>mba essay writing services</a>
essays help

AshlenKlmr , 23 Sep 2022

essay writing service online
<a href=https://bestsessays.org>cheap essay writing services</a>
help writing essays

CharitaKlmr , 25 Sep 2022

essay writing services scams
<a href=https://buy-eessay-online.com>help write essay</a>
best essay for you

ChelsaeKlmr , 26 Sep 2022

essay writer website
<a href=https://buytopessays.com>online essay writers wanted</a>
custom essay writing sites

PennyKlmr , 27 Sep 2022

help writing argumentative essay
<a href=https://cheapessaywritingservice1.com>scholarship essay writing service</a>
cheap custom essay writing services

TammieKlmr , 28 Sep 2022

the best essay writers
<a href=https://customcollegeessays.net>english literature essay help</a>
i need help with my college essay

AntonieKlmr , 29 Sep 2022

personal essay help
<a href=https://customessays-writing.org>help me with my essay</a>
buy essays online cheap

RhiamonKlmr , 30 Sep 2022

help me write essay
<a href=https://customessaywwriting.com>essay buy</a>
custom essay writing reviews

CharoKlmr , 30 Sep 2022

i need help with my college essay
<a href=https://customs-essays-writing.org>best custom essay writing</a>
best cheap essay writing service

DronaKlmr , 01 Oct 2022

essay writing on customer service
<a href=https://firstessayservice.net>essay editing services</a>
buy cheap essay

TwylaKlmr , 02 Oct 2022

help me write a compare and contrast essay
<a href=https://geniusessaywriters.net>professional custom essays</a>
essay customer service

LeilahKlmr , 03 Oct 2022

rutgers essay help
<a href=https://howtobuyanessay.com>essay introduction help</a>
essay writers for hire

CthrineKlmr , 04 Oct 2022

need someone to write my essay
<a href=https://lawessayhelpinlondon.com>admission essay help</a>
essay help 123

GinnieKlmr , 09 Oct 2022

essay writing service
<a href=https://ukessayservice.net>do my essay for cheap</a>
mba essay writing services

VivieneKlmr , 12 Oct 2022

cheap custom essay
<a href=https://writemyessaycheap24h.com>original essay writing service</a>
college essay services

EasterKlmr , 13 Oct 2022

essay community service
<a href=https://bestcampusessays.com>essay writing service toronto</a>
writing custom essays

DorolisaKlmr , 19 Nov 2022

analysis essay help
<a href=https://bestessayreviews.net>fast essay writing service</a>
helping others essay

MerolaKlmr , 22 Nov 2022

civil service essay
<a href=https://bestessaysden.com>write my law essay</a>
need help in writing an essay

AshlenKlmr , 23 Nov 2022

essay title help
<a href=https://buyacademicessay.com>do essay writing services work</a>
buy pre written essays

NaniceKlmr , 26 Nov 2022

write my admission essay
<a href=https://buytopessays.com>custom writing essays</a>
college application essay service

PennyKlmr , 29 Nov 2022

coursework sample
<a href=https://brainycoursework.com>buy coursework</a>
coursework samples

InessaKlmr , 23 Jan 2023

Leave a comment

Collaborative partners

This blog is supported by the Arts and Culture section of the Spanish Embassy in Belgium and by the Brussels section of the “Instituto Cervantes”, under the SciComm initiative #SPreadScience.

Instituto Cervantes Arts and Culture section of the Spanish Embassy

Disclaimer: The content of each post in “A Spoonful Of Science” is the responsibility of the corresponding author(s). Therefore, the viewpoints expressed on the blog are those of the author(s) of each post, which do not necessarily reflect the viewpoints, thoughts and opinions of CEBE members and representatives.

Would you like to send us a question?

“Do you have a burning question that you would like a scientist to answer? Or a topic you would like to learn about? Send it to us!”


Science Chronicles (19 Blogs)
CEBE answers (13 Blogs)
Gender equality in science (7 Blogs)
Winners "Your research in one image or 1000 words" - 1st Edition (7 Blogs)
Webinars (5 Blogs)
CEBE's online events (1 Blogs)