# The colour of sky is blue

1. Aug 11, 2011

### shiva999

is that true that the colour of sky is blue or is it just the reflection of sea water

2. Aug 11, 2011

3. Aug 11, 2011

### jewbinson

I've jsut read that article and it's very easy to understand.

I think the sea is blue because salt water is light blue and the size of the sea causes an accumulaton of light blue, which is dark blue. This is just a guess though.

The sea is often much darker than the sky so I don't think the sea is a reflection of the sky

4. Aug 11, 2011

### ZapperZ

Staff Emeritus
Salt water is light blue? Since when?

Try this at home!

Zz.

5. Aug 11, 2011

### jewbinson

You can't see it in just a few cm^3 or m^3 of water. But perhaps in 10^6 or 10^8 m^3 of water you will begin to notice a pale blue. As I say, it is very, very pale.

Aha! Look what I found

http://en.wikipedia.org/wiki/Color_of_water

6. Aug 11, 2011

### jewbinson

So not just salt water, pure water is blue

7. Aug 11, 2011

### ZapperZ

Staff Emeritus
You have to be careful and understand what you are reading. There's a difference between the color of water seen through a transmission of light versus the reflection of that light. The FACT that the ocean's "color" can CHANGE, depending the time of day, the color of the sky, etc.. etc. (try sitting by the ocean for a long time and under different circumstances) means that it has very little to do with the color of the water.... unless you think the color of the water itself changes that often.

Zz.

8. Aug 11, 2011

Right, so water is blue. But that's still not why the sky is blue.
Also, we can't entirely disregard the fact that part of the reason why large masses of water such as seas look blue is because of the reflection of the sky's colour.

9. Aug 11, 2011

### jewbinson

But the sky's blue colour tends to be lighter than the colour of the sea (apart from seas in exotic Islands which can be very light blue).

When the sky is cloudy, the sea still tends to be quite dark blue.

10. Aug 11, 2011

I said 'part' of the reason, not the main reason! :-)
The question here was why the sky is blue, so I think we've got that solved well.

11. Aug 11, 2011

### thebiggerbang

To debunk the OP, you can just assume that the guy observing the sky is sitting in an African dessert. Miles away from water! But the sky is still blue there! It's all about scattering!

12. Aug 11, 2011

### jewbinson

What does there being no sea have to do with the sky being blue? The converse clearly has some relevance, because the sea reflects light. But the sky does not. Unless you were thinking about the sea "omitting blue light and the sky trapping the blue light". But that's got quite a few flaws to it

13. Aug 11, 2011

### ZapperZ

Staff Emeritus
But that already debunks even YOUR suggestion. I mean, how come the color of "salt water" changes when the sky is cloudy? Yet, one can easily argue that a "cloudy" sky causes the reflection to darkens.

At some point, responses to posts on PF must be based on more than just guess work. Re-read the https://www.physicsforums.com/showthread.php?t=414380" that everyone has agreed to, especially our policy on speculative post. If not, this thread is in danger of being locked.

Zz.

Last edited by a moderator: Apr 26, 2017
14. Aug 11, 2011

### klimatos

Obviously the color of the sky varies from day to night and also with changes in the weather.

That said, the blue color of clear daytime sky is a result of the shorter wavelengths of sunlight being scattered by the molecules of the air itself (Rayleigh scattering). The scattering takes place all over the sunlit hemisphere of the atmosphere; hence the blue light appears to come from all parts of the sky.

Air molecules are too small to reflect other light wavelengths, but larger atmospheric particulates can do this. This gives us the common reddish sunsets.

15. Aug 11, 2011

### Dr_Morbius

Sunsets are red for the same reason that the sky is blue. When you look at the sun near the horizon the atmosphere preferentially scatters the blue light away from your eye more than the red light. That makes the sun look redder than usual. The affect is increased by the fact that when the sun is near the horizon you're looking through more atmosphere than when it's overhead.

16. Aug 11, 2011

### LostConjugate

I can't take it any longer! Someone just tell jewbinson that the ocean is blue because the sky is blue.

17. Aug 11, 2011

### Staff: Mentor

I don't think that's true though.

18. Aug 11, 2011

### LostConjugate

Well yea at sunset the ocean is red.

19. Aug 11, 2011

### jewbinson

???

This simply isn't true. When the sky is cloudy, is the sea white?

At night, there is nothing to light up the ocean, and water does not emit light, so obviously you see black (apart from the reflection of the moon)

20. Aug 11, 2011

### LostConjugate

The sea should be less blue if your part of the sky is covered in thick clouds, yes. Some blue still scatters from where the sky is blue perhaps.