# Why is the sky blue again?

1. Jun 11, 2006

### vincikai

okay, i forgot why is the sky blue again?

2. Jun 11, 2006

### Danger

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!

3. Jun 11, 2006

### gnomedt

4. Jun 11, 2006

### 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/

Last edited: Jun 11, 2006
5. Jun 11, 2006

### Gokul43201

Staff Emeritus
What is your current educational level?

6. Jun 11, 2006

### dav2008

It's blue again because the sun rose this morning.

7. Jun 11, 2006

### DaveC426913

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?

8. Jun 11, 2006

### 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..."

Last edited: Jun 11, 2006
9. Jun 11, 2006

### 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"

10. Jun 11, 2006

### Staff: Mentor

11. Jun 11, 2006

### 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]

Last edited by a moderator: May 2, 2017
12. Jun 11, 2006

### Farsight

13. Jun 11, 2006

### Hootenanny

Staff Emeritus
Wait, you can see UV light?

14. Jun 11, 2006

### Farsight

Last edited: Jun 11, 2006
15. Jun 11, 2006

### Hootenanny

Staff Emeritus
Nope, you cannot see ultra violet light. However, you have see visible violet light - $\lambda ~ 400nm$.

Last edited: Jun 11, 2006
16. Jun 11, 2006

### Hurkyl

Staff Emeritus
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:

Maybe it's more than merely "like" that. :tongue:

17. Jun 11, 2006

### Farsight

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

18. Jun 11, 2006

### Hootenanny

Staff Emeritus
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.

19. Jun 11, 2006

### 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".

20. Jun 11, 2006

### Hurkyl

Staff Emeritus
Interference bows aren't ultraviolet light.