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

It’s the mother of all questions!

Well, except maybe What’s the Meaning of Life? -- which we actually have our own scientific

answers to …

Or What Is Love? -- We have some thoughts on that, too.

But 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.

Why is the sky blue?

Just give me a couple minutes and I'll let you know why.

This is the World’s Most Asked Questions!


The sky is blue because of three things: light from the sun, particles in our atmosphere,

and the fact that you’re a human.

I’ll get to that last part in just a minute.

But 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 wavelengths. Light with the

longest wavelengths are at the red end of the spectrum, and light with the shortest

wavelengths are at 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


This effect is known as Rayleigh scattering, because it was first described by John William

Strutt, aka the Third Baron Rayleigh, who in 1871 published his mathematical proof that

blue wavelengths of light in the atmosphere are scattered 16 times more than red wavelengths.

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.

So SciShow viewers are in luck, because when we surveyed you, the most popular favorite

color from the rainbow was blue. 33.55% of survey takers reported blue as their favorite

color, while yellow was the least favorite at only 2.8%. Poor yellow.

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 they like yellow. And maybe the sky

looks completely different to them.

Of all the fascinating questions in the world, what question do you want answered most? Let

us know on Facebook or Twitter or in the comments down below, and we will try to answer those

questions in a new video at the end of the month. And don’t forget to use hashtag #WMAQ

and stay tuned for more answers to the World's Most Asked Questions.