Why does the sky look blue?

Hi Guys!

Today, I’ve got an awesome physics for you’ll. Hope you’ll enjoy!

Why can’t I eat chocolates for breakfast? Why is the sky blue? Whenever I asked these questions as a child, I was told “it just is”. I’ll leave you to do the chocolate one but today I’m going to explain, why the sky is blue or rather looks blue.

To start off, I’ll give you a few pieces of the jigsaw puzzle and then, I’ll put them all together.

So…..why is the sky blue?

Puzzle Piece 1 – Electromagnetic Waves
The Electromagnetic Spectrum essentially accounts for the entire range of light. From radio waves to gamma waves, they are all around us but most of them are invisible to us. All light in the spectrum is a wave of alternating electric and magnetic fields, hence the name. Interestingly, the only thing that sets these waves apart is their wavelength and in turn the frequency and energy.

The Electromagnetic Spectrum

What if I told you that you have a specialised instrument to sense electromagnetic waves and it’s within a foot of you? Buuuutt, it’s not electronical, it’s biological and you have 2 of them. Yup, they’re your eyes. Your brain and eyes helps you translate different wavelengths into colours. While most of the spectrum may not be visible to us, some animals like insects and snakes can see infrared which helps them sense body heat and hunt for prey, while others like butterflies and bees see UV rays which helps them navigate.

Puzzle Piece 2 – The Sun-

The sun blasts the whole electromagnetic spectrum (except gamma rays) but 99% of it is UV Rays, Visible Light and Infrared. In fact, the sun produces energy of about 1 trillion nuclear bombs a second. If you’ve ever seen white light zipping through a prism (or a picture of it), you know that white light contains all the colours of the visible spectrum. So, the sun is white because it a emits all the colours but it appears yellowy orange for the exact reason that the sky appears blue (which I’m building up to).

Puzzle Piece 3 – Visible Spectrum Stuff –

White surfaces reflect all the light, and black surfaces absorb all of it. It is also important to understand the colours that you see are the colours of the light that are reflected back to your eye. So plants look green because they don’t need green light and they reflect that.

Puzzle Piece 4 – The Sky/Atmosphere –

Our atmosphere creates friction. That’s why asteroids burn up when they come into our atmosphere except the really big ones (RIP Dinosaurs) but the moon has no atmosphere and thus has several craters. Most of our air is composed of nitrogen (78%) and oxygen(21%).

Piecing the Puzzle Together –

So, our sun blasts almost the entire electromagnetic spectrum which after 8 mins and 23 seconds, reaches Earth. Most of the UV gets filtered out at the ozone layer. The infrared provides us with heat. Light travels in a straight line and doesn’t change direction unless it hits something.

Most light just goes through light particles like oxygen and nitrogen but shorter wavelengths like those of green light (just somewhat), blue light and violet light get absorbed and scattered in multiple directions. It’s like a bunch of ping pong balls get hit by bunch of paddles where the light is ping pong balls and oxygen and nitrogen are the paddles. Since blue light scatters a lot, the sky looks blue. This scattering of light is known as Rayleigh Scattering ( Quick Note- This is different from refraction as refraction explains the behaviour of light when it changes medium and this entire process occurs only in air ( gas) ).
However, this begs the question, why is the sky not violet as violet light has a shorter wavelength than blue light and scatters more. There are 2 primary reasons for this:

  1. The sun emits more blue light than violet light.
  2. We have 3 types of cones (parts of the eye responsible for colour vision) in our retina, Red, Green and Blue so our eyes are more sensitive to blue light.
The spectrum of the rays than the sun emits showing that it emits more blue light than purple light.

This scattering also explains why the sun looks like a mix of yellow, orange and red. Light of shorter wavelengths scatter more but light with longer wavelengths don’t scatter much and reach our eyes directly so the sun and it’s surroundings appear a yellowy orange. Now, that we’ve got this model, we can expand upon it. For example, why is the sky shades of yellow, orange and red in the evenings. This is because during sunrise and sunset, the sun light has to travel through more of our atmosphere and the shorter wavelengths like blue green and violet light get scattered before they reach our eyes so we see the skies that are a mix of red, orange and yellow.

Light has to travel through more of our atmosphere during sunsets than noon as depicted in this picture

To summarise,

The sky, after all, is just transparent air, “a stage upon which all colours of light dance.” Once light enters the atmosphere, the different colours dance differently as they bump into the air.

Red light with a longer wavelength moves relatively straight. Green light dances a little more randomly. The shorter blue and violet wavelengths get scattered by the air, dancing all around, making the sky around us appear blue. The remaining red, yellow, and orange wavelengths mostly pass through and hit our eye at once ; making the sun, in turn, appear yellowish .

Remember the blue light is a result of the interaction between the sunlight and air. This means that on a different planet, where the atmosphere is composed of different gases, the sky may not be the blue sky we see. So, the sky as seen from the moon looks black because…..No atmosphere, no gas, no gas, no light scattering. No blue.

That’s just a quick comic depicting a noon sky on the left and sunset on the right.

Drop any doubts in the comments and I’ll answer ASAP. Also, if you guys have any questions related to any of the branches of science that you’ll want me to answer, drop them in the comments. Thanks for Reading and until next time :)!!



43 thoughts on “Why does the sky look blue?

      1. Reflection. Or something like that. Not refraction. And the part about sunsets being further away so the light gets scattered before it reaches us is awesome.

        Liked by 1 person

      2. Oops, my bad, I should’ve made it clearer that scattering is not refraction all though it may seem similar. While refraction can be considered a type of scattering, the scattering that gives the sky its blue isn’t refraction.
        One reason is that refraction is a term used to describe the behaviour of light when it changes mediums whereas this scattering occurs in the air (gas) and there is no change in medium.
        Hope this helps,,,,,maybe I should do a post on this?
        Hope this helps
        Yeah, everything about light and sunsets is awesome!!
        💜(indigo coloured heart)💙💚💛🧡❤️ = 🌈…sorry, i’m bored……lockdown does this to you sometimes.

        Liked by 1 person

  1. This is the kind of things we need. Something which makes us feel better, learn something or at least remember about it. 🙂 And the ‘Why is the sky blue? Whenever I asked these questions as a child, I was told “it just is”.’ reminded me of one of my grandfathers. He also used to had such questions, but people didn’t care, so he drank a lot of alcohol and felt left alone with his thoughts. So thanks from my heart for this post. My grandfather would have loved this one, I bet. 💜 Thanks for having people like you.

    Liked by 1 person

    1. Glad you learned something and enjoyed this post…
      I’m so sorry for your grandfather, too many thoughts can often just blow up in your head and get overwhelming so it always helps to speak to someone or even just pen them down.
      Thanks for all your awesome wishes!
      💙💙

      Liked by 1 person

  2. “Tell me why the stars do shine.
    Tell me why the ivy twines.
    Tell me why the sky’s so blue,
    and I will tell you why I love you.”

    lyrics to an old song

    Very interesting, dr universe. Love this post! Thanks to Eliza for reposting! Cheryl

    Liked by 1 person

      1. yes rhyme comes easy to me … I watch the sky each day and only the clouds give it a different hue … the water, it can change colour within a few moments or metres 🙂

        I’m more concerned about pollution 🙂

        Liked by 1 person

  3. Hey dr universe, I know I’m super late in reading this post but I found it so interesting!! I’m not really a fan of science cause I find it too complicated, but you made it super easy to understand and I really enjoyed reading it! Especially how you divided it into the different puzzle pieces, that was super useful…and while reading the comments (all of them, like an unbelievably bored person) I have a small request… can you write a post about stars, like their lifespans and how they’re born, something along those lines? They fascinate me, and it would be awesome if you could shed some light on that (lame pun alert) 🙂

    Liked by 1 person

    1. Thanksssss!
      Yayy! Mission accomplished, it’s awesome that you enjoyed (and understood) it.
      I do that ALL the time….I guess boredom is running rampant these days
      Stars…sounds like a pretty bright topic to write about-I’ll most definitely write that one soon so keep an eye out for it (it’s already sitting accumulating gas and dust as everything starts falling into place).
      Sidenote – Since we’re into puns, I have included 2 in here, one obvious and one which is slightly harder. Lemme know if you get them after reading the post.
      😉🙃

      Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s