Why is the Sky Blue?

World’s Most Asked Questions: Why is the Sky Blue?

It’s the mother of all questions! One of the most googled questions in the English-speaking world is the one that kids are famous for asking – and grown-ups are notorious for avoiding, because they just don’t know.

So why is the sky blue??


The sky is blue because of three things:

1.Light from the sun

2.Particles in our atmosphere

3.The fact that you’re human

Let’s start with how sunlight interacts with particles in the sky. I’m not talking about dust or water droplets or other big particles, which are sometimes mistakenly used to explain the sky’s blueness.

Instead, it’s the very small particles in our atmosphere that make the sky look blue. I’m talking about molecules of the air itself, mostly molecular oxygen and nitrogen – O2 and N2.


These molecules are super-abundant in our atmosphere and so small that they’re even smaller than wavelengths of visible light! And that’s the key. Visible light consists of a range – or spectrum – of different wavelenghts.

Light with the longest wavelengths are at the red end of the spectrum, and light with the shortest wavelengths are the blue and violet end.


Since the blue wavelengths of light are shorter, they’re more likely to strike those tiny molecules of oxygen and nitrogen more often, and when they do, they get scattered in all directions.

But violet light has the shortest wavelength of all visible light, and it’s scattered even more strongly by tiny particles – so why isn’t the sky purple?


Well, this is where being human comes into play. Our human eyes perceive the middle of the spectrum most sharply which is why blue is way easier to see than violet. So even though violet light is being scattered around like crazy, we just don’t detect it as well as we detect the blue.

And that’s why the violet band of the rainbow is often the hardest for us to make out.

But, there are other animals that can see all the way into the ultraviolet end of the spectrum, like honeybees and some birds.



So maybe the sky looks completely different to them??


16 thoughts on “Why is the Sky Blue?

  1. After a couple of weeks beneath a searingly blue Spanish sky, it’s hard to believe that its actually purple. It reminds me of those old Kodak Instamatic photos I took in the 1970s that were returned un-printed because the sky came out purple.

    Liked by 1 person

  2. Pingback: Why Is the Ocean Blue? | Science time

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