Why is the sky blue?

“Why is the sky blue?” is a natural question, asked by many people, children and adults alike. In this video, Fermilab’s Dr. Don Lincoln answers the question and even reveals the surprising fact that it should really be purple.

Fermilab physics 101:

Fermilab home page:

[doggedout]

If I asked an adult why the sky is blue and they answered: "Because the ocean is blue it reflects the light from the sun....": I don't think I would ask them any more questions.

[Ambienfinity]

Stephen Hawking said something similar: he liked hearing what children asked because they didn't know enough NOT to ask the big questions.

[banehog]

Fun fact, the sky is *not* blue, it is cyan. What you call "indigo" in the video is the actual color blue. Cyan sits somewhere between blue and green. The arrows at 3:37 should therefore read "red, orange, green, cyan, blue and violet".

[PeterGaunt]

Excellent, Don! This is the sort of thing I used to head down to the local library to find out in the late 1950s and 1960s. No Internet! When I got my first digital camera around 2002 and had to work out how to stop blue skies going purple. Now I know just why they went purple. Thanks!

You should do more of these 'childhood-type' question videos if only to remind us where we came from. One of my earliest memories of working something out was realising aged two or three that wind wasn't caused by the trees waving around.

[AlexisVG]

Congrats for explaining such a common topic and making it sound so simple.

[rumim]

Finally more Don Lincoln good physics content

[sapelesteve]

I must say that that was quite an illuminating video Dr. Don! 👍👍😉😉 Good to see you back!

[Jaggerbush]

For once - I kinda knew this … I’m excited for new uploads. I wish you had a ton of Fermi paradox ones.

[nexus]

Could you please make a video on "Why scattering occurs?"

[paulthompto]

Love all your submissions, Simply put - You are the best at explaining everything from Quantum Mechanics to the scattering of sunlight😀

[VapidSlug]

My gosh, I told my daughter it was because of Rayleigh scattering. I was sweating a bit during this video but turns out I didn't misguide too much.

[joseraulcapablanca]

Thanks Doctor Lincoln. I remeber from school that critical opalesence was mentioned but cannot remeber what it is and if it plays a role here. I am sure you know. Keep up the good work

[juzoli]

“Air is blue” is NOT an incorrect answer.
In general, the color of ANYTHING is defined by what wavelengths that thing reflects, refracts and absorbs. For a leaf, this adds up to green. For blood, it adds up to red. For sky, it adds up to blue

And the explanation of HOW the sky is blue is the pretty much the same explanation of how anything else gets its color.

[peterkelley]

I've heard this before, so I listened to see if anything had changed. The biggest change was that the human eye is imperfect at seeing the purple spectrum of light.
I was expecting Dr. Lincoln to just walk off the screen with the VERY first explanation. and walk back on from the other side to finish the video!

[AviatorJ]

Good video explaining the colors in the sky...but that cute little girl near the end of the video! <3

[cmilkau]

i think I got the sunset thing, but it is quite confusing at first (scattering blue light makes a blue sky, except for sunset, then scattering even more blue light makes the sky orange. what?). If you can see a picture from space, it helps. The blue being scattered means it can't penetrate as deep, as it goes in all kinds if directions rather than away from the sun.

[JohnGalt]

My Physics teacher use to say, we only explain "how" not "why". This is How the sky is blue, not why.

[ckmishn]

"Wait, is the sky really purple?"

Well, when I get a purple haze in my brain I do need to be excused while I kiss the sky. Make of that what you will.

[duanjianxiong]

"air scatters blue light more, so sky looks blue".
"at sunset, light goes through more air so more air scatters blue light even more, so sky looks orange/red"
scratching my head.

[amalieemmynoether]

I always feel there's a step missing in the explanation of Rayleigh scattering. If blue (and violet) light is being scattered more, then wouldn't it be scattered away from the observer? This would suggest that the sky should always be orange/red not just at sunrise and sunset.
I've never found a decent explanation to fill in that gap between blue light scattering and more blue light reaching the observer.