# Why is the sky blue again?

#### vincikai

okay, i forgot why is the sky blue again?

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#### Danger

Gold Member
Scattering. The refractive index of the atmosphere tends to spread the bluish part of the spectrum around, while blocking other parts of it. That's a pretty sad explanation, but I hope that it will hold you until the big guns come around. Welcome to PF!

#### Farsight

The sky is blue because we kind of live at the bottom of a rainbow, only it's an "atmosphere bow".

Look at the colours on a rainbow. Blue is at the bottom. That's been refracted most. You can see the other colours that get refracted less at dusk or dawn. Sometimes you can even see what's called "the green flash".

http://mintaka.sdsu.edu/GF/

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#### Gokul43201

Staff Emeritus
Gold Member
vincikai said:
okay, i forgot why is the sky blue again?
What is your current educational level?

#### dav2008

Gold Member
vincikai said:
okay, i forgot why is the sky blue again?
It's blue again because the sun rose this morning.

#### DaveC426913

Gold Member
Farsight said:
The sky is blue because we kind of live at the bottom of a rainbow, only it's an "atmosphere bow".
I love this logic!:rofl:

So, how high would you have to climb for the sky to be green or yellow?
If we dug a hole, and looked up from the bottom, would we see an Indigo or Violet sky?

#### Farsight

Dave, it's a nice simple analogy. Hits the spot, you know?

http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html

"If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength. The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high atmosphere, so there is less violet in the light. Our eyes are also less sensitive to violet. That's part of the answer; yet a rainbow shows that there remains a significant amount of visible light coloured indigo and violet beyond the blue. The rest of the answer to this puzzle lies in the way our vision works..."

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#### disregardthat

dave got a pretty good point i think. Isnt the sky blue because the air molecules reflects mostly blue light? that will say: lightbeams with the wavelength that we see as the color "blue"

#### Doc Al

Mentor
Farsight said:
That's a good link--read it. But the blue sky has nothing to do with us "living at the bottom of a rainbow", whatever that might mean.

#### Farsight

Have a look at this picture of a rainbow, Doc. The blue is the bottom stripe. The red and yellows of dawn/dusk are at the top. And there isn't much green.

http://www.sundog.clara.co.uk/rainbows/images1/shim18.gif [Broken]

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#### Hootenanny

Staff Emeritus
Gold Member
Farsight said:
Interestingly you can also see the ultra violet.
Wait, you can see UV light?

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#### Hootenanny

Staff Emeritus
Gold Member
Farsight said:
Yep. Look at the photo above.
Nope, you cannot see ultra violet light. However, you have see visible violet light - $\lambda ~ 400nm$.

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#### Hurkyl

Staff Emeritus
Gold Member
Farsight said:
Yep. Look at the photo. There's a half-width band of pale translucent yellow under the indigo. With a real rainbow you have to turn your head sideways before you can see it.
I zoomed in on the image as close as I can with my browser, and I can't find any yellow pixels below the violet band.

Of course, I didn't expect to see any UV light anyways, since my monitor displays colors with a mixture of red, green, and blue. :tongue:

Farsight said:
It isn't obvious, it's rather like an "after image" colour when you close your eyes after looking at a bright light.
Maybe it's more than merely "like" that. :tongue:

#### Farsight

Look at the photo Hootenanny. And take a sidelong look at a rainbow next time you see one. It ain't violet.

#### Hootenanny

Staff Emeritus
Gold Member
Farsight said:
Look at the photo Hootenanny. And take a sidelong look at a rainbow next time you see one. It ain't violet.
I don't disagree that the photo shows violet light. However, I disagree with the fact that you said it was UV light, UV light is outside the visible spectrum, therefore we cannot see it. We can however, see visible violet light. In addition, I have seen a rainbow with a purple stripe before.

#### Farsight

You can't actually see a colour. All you can really see is a brightness inside the violet, but you have to use the side of your eye. There is definitely some light in this region where the UV ought to be. Search google on "interference bows".

#### Hurkyl

Staff Emeritus
Gold Member
Interference bows aren't ultraviolet light.

#### dav2008

Gold Member
Hootenanny said:
I don't disagree that the photo shows violet light. However, I disagree with the fact that you said it was UV light, UV light is outside the visible spectrum, therefore we cannot see it. We can however, see visible violet light. In addition, I have seen a rainbow with a purple stripe before.
Exactly. By definition you can't see UV light.

#### Farsight

Interference bows aren't ultraviolet light.
I know, hurkyl. But have a look again for some yellow pixels. You will find them. That's step one.

By definition you can't see UV light.
Sure it's beyond the normal colour range Dave, and sure, you can't normally see it. But not by definition.

#### Hootenanny

Staff Emeritus
Gold Member
Farsight said:
Sure it's beyond the normal colour range Dave, and sure, you can't normally see it. But not by definition.
Go to google and type in 'define: UltraViolet light'.

Ultraviolet light has a wavelength of $100nm < \lambda < 400nm$, the shortest visbible wavelength of light our eyes can percieve is 400nm therefore, I repeat again UV light is not visible to the naked eye under any cicumstances

#### ZapperZ

Staff Emeritus
2018 Award
Farsight said:
Sure it's beyond the normal colour range Dave, and sure, you can't normally see it. But not by definition.
Under what "abnormal" condition are you proposing that YOU can "see" UV light?

Zz.

#### Hurkyl

Staff Emeritus
Gold Member
Farsight said:
I know, hurkyl. But have a look again for some yellow pixels. You will find them. That's step one.
If you know that interference bows are not ultraviolet light, then why the heck did you bring them up?

Farsight said:
Sure it's beyond the normal colour range Dave, and sure, you can't normally see it. But not by definition.
You might want to check the definition again. e.g. the first three Google definitions, or the first sentence at Wikipedia.

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