Why is the Sun Yellow and the Sky Blue?

And what if I told you the sky isn't really blue?

Created by Henry Reich
Thanks to Nima Doroud for contributions.
MinutePhysics is produced by Neptune Studios LLC

MinutePhysics provides an energetic and entertaining view of old and new problems in physics – all in a minute!

[BennyOcean]

By far the best explanation I've ever seen of this.

[HaydenII]

I'm going to need to watch this twice.

[DeadUnicornClub]

This video made me sad.  I can only perceive 2.3% of electromagnetic spectrum and so I can only enjoy a fraction of the grand ballet of light.  :(

[erikziak]

An example of how the scattering (and the curvature of the earth) affects visibility:

Both images were shot from Koliba at Kamzík in Bratislava with the same lens, just with different cameras (one converted to near IR)

The first image is shot in "visible light" (give or take 390 - 720 nm wavelength):

This second image is shot in the near infrared (about 720 to 900 nm):

See the difference? While visible light showed up to 55 km to the cooling towers of the Bohunice Nuclear Power Plant, the IR spectrum showed the Považský inovec mountains, about 100 km away. The atmosphere did scatter all the short wavelengths of (reflected) light of the visible spectrum from far away, but the longer near IR light passed, or at least a part of it. IR is less affected by Rayleigh scattering, Mie scattering, by mist and by haze than all colors of visible light (the entire visible light range). That is why the IR image shows also the distant mountains. And no, you can try to boost contrast on the visible light image as far as you want, there is nothing there (I tried it when processing the RAW files, believe me).

Another funny thing: Notice the sudden drop in contrast in the IR image at the level of the nuclear power plant? You might expect that this line forms the horizon behing which we cannot see due to earth curvature. We can find out:

The picture was taken from a height about 400 meters above sea level and the power plant is about 175 meters above sea level. Taking the elevations I wrote here (got them from a hiking map) and using basic geometry maths taking into account also the refraction of the atmosphere (used the values defined for visible light also for near infrared as there is little difference), the calculations say the horizon (or the nuclear power plant on the horizon) should be 58, 5 km away. This nicely fits with the measured distance of 55 km using google maps. It is so nice when things work out!

But how do we see the mountains in the background? Well, they are high, reaching up to 640 - 900 meters in height, so they "rise" behind the horizon. Basically the IR photo shows everything up to the horizon at a distance of about 58 km and then the background at a distance of roughly 100 km. You cannot see anything between the horizon and the hills due to earth curvature.

But you would not see the background anyway as you are limited by your eyes to the part of electromagnetic spectrum that we call visible light. The visibility was rather very good when I took the picutres, the conditions are often much worse, so you cannot see even the cooling towers of the nuclear powerplant. Even the visible light image taken at a rather very good visibility conditions has just a faint picture of them.

Next time I will try to "see" (using both visible and near IR) the Apls in Austria from Devínska Kobya. It should also be about 100 km. And I should also see Vienna, just on the horizon.

[Teck_]

I enjoyed that last metaphor more than I should have...

[retracian]

Could have used this video back when I was a kid and nearly went crazy trying to figure out why the sky was blue

[seane]

I remember the sun being yellow in the 90s and early 2000s anyone else? But now its white...

[MarioFanGamer]

Anyone who asks why the sky should be sometimes green at some point: It really gets on some sunrises and -sets even though just partly but still existing. The lower the sun is on the horizon, the more colours of shorter wavelength gets filtered near the sun. In fact, green is one of the first/ last colours aside from blue/ violet you see at a such sunrise/ -set.

[durdentyler]

0:39 you say its blue because its scattered and ends up in your eye, but that doesnt tell me why its blue, and then you take your time explaining why things appear yellow, honestly this left me with more questions than answers.

[Falcrist]

"Is the sky blue?"
*Yes!* In fact blue is actually defined as the color of the sky in most dictionaries.
Remember, color is NOT defined by the frequency of light, but by how we *perceive* it.

[orange]

The sun at noon day in the 1970's was very yellow and you could observe it briefly without harm. In recent years the sun at noon day is now a blinding ultra white. Something has changed be it in the sun, our atmosphere or magnetic field. This change is very obvious but there has been no press on the subject. This fact is key to many things going on such as the controversial global warming. What is disturbing though is that barely anyone has noticed this blatant change in the suns appearance even though it's right in your face everyday. Do people not think anymore, it should shake you up if you didn't notice this change. You should be asking yourself what's wrong with me and then reevaluate the world around you. Come on people wake up and get a clue.

[electronicsNmore]

I always thought it was the sunlight interacting with gases in the atmosphere.

[GuitarHeroPhenomSux]

But how can mirrors be real if our eyes aren't real? 

[tjadadada]

Just the kind of video I subscribed for, quite a long time ago. Thanks a lot.

[mattweeeee]

6 years ago the sun was yellow. Now its white

[caelankelley]

So why don't we ever get green skies? Shouldn't we see green as the sky transitions from blue to red? 

[randydaignault]

I really enjoy minute physics because of the basic straight forward self in which it is presented .

[Robin-qfdp]

Thank you for making such awesome videos, I love minute physics :)

[uncinarynin]

Interesting implication for artists: As the clear blue sky is a source of diffuse light (going in all directions), shadows aren't all dark (as they are on the moon, where anything in the shadow is only illuminated by starlight). On a clear sunny day, objects in direct sunlight appear slightly yellowish, the same objects in shadow appear blueish because the main light source in the shadow is the blue sky. Take this into account when creating a colour painting; you get a very sunny effect by not only adding hard dark shadows but also by making the shadows blue and adding a slight yellow tinge to what is in sunlight.

On a cloudy day however, all light is diffuse, the sky is grey, there are no hard shadows and those don't have a colour.

[Hereson]

His voice is just soo relaxing.. Please become my teacher at my university ;_;