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Why Water Blue?

  1. Sep 27, 2006 #1
    SkyBlue=WaterBlue? No!

    Hey guys, I always thought that water is blue because it reflects the color of the sky. Today when I was surfing through the web I found this webpage that says that water is blue because it really IS blue! Check it out:

    Is this true? What do you guys think?
    Last edited: Sep 27, 2006
  2. jcsd
  3. Sep 27, 2006 #2
    Ah. Colour. Now that's an interesting one. Let's talk about air first:

    The sky is blue because of scattering. The blue shorter-wavelength photons interact with electrons and bounce off sideways more than the red longer-wavelength photons. So when you look up at the clear blue sky what you're seeing is more of the blue photons. And when you look at the setting sun you're seeing more of the red photons. But this doesn't mean air is blue, because we know it's colourless and transparent.

    Anyhow. Water is like air. Imagine you're swimming underwater in the middle of the ocean. You get this self same scattering thing going on. So when you look into the distance you see blue. But the water is colourless, transparent. It isn't really blue.

    But: nothing's blue really. Blue is just our perception of some frequency band. And if that's what we perceive, maybe it's simpler to say what the hell, if it looks blue, it's blue.

    Don't anybody mention violet. Or rainbows. And don't anybody ask what colour is the oil in that puddle.
  4. Sep 27, 2006 #3

    Claude Bile

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    It is tough to make a judgement on whether the author is correct because saying something 'is Blue' is a rather ambiguous statement. He doesn't say exactly what he is referring to, the absorption spectrum?, scattering spectrum?, reflecting spectrum?, a combination of these? It is hard to say he is wrong, because we certainly observe water to look blue, to some degree he is skirting the issue of WHY the sky is Blue.

    There is an aspect of this scenario here that is often overlooked (and this goes for air as well as water), and that is the response of the human eye to different wavelengths. We observe the sky and the ocean to be blue (as opposed to say, violet), because our eyes are strongly receptive to blue, but relatively weakly receptive to violet.

    We observe the sky and ocean to be blue primarily due to a combination of how light scatters and the natural response of the human eye.

  5. Sep 27, 2006 #4
    Last edited: Sep 28, 2006
  6. Sep 27, 2006 #5
    I agree. But still, there is still a big difference between water being blue because it reflects the blue wavelength of light from the sky and water being blue because of its special properties.

    I mean the point is that even if our sky "looked" green, water would still "look" blue right?

    That's interesting. I didn't know that.
  7. Sep 27, 2006 #6


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    To answer that question just look at the water on a overcast day.
  8. Sep 27, 2006 #7


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    Okay, that's the third time that I've tried that link, on 3 different computers, and it always comes up with 'page cannot be found'. :grumpy:
  9. Sep 28, 2006 #8

    I'm awfully sorry Danger. I copied and pasted, and picked up the abbreviated version. I've mended the link above. It's on:

  10. Sep 28, 2006 #9
    I can't wait that long. :zzz: Can anyone please just tell be the answer?
  11. Sep 28, 2006 #10


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    As a diver, I can tell you that the deeper you go, the less reds and yellows you will see. Below about twenty feet, there are no reds and yellows at all. If you turn on a flashlight, which has comparatively very red light, but is in fact simply white(ish) all those colours will come leaping back.

    Water preferentially absorbs reds, while blues can reach farther down.

    The colour of water has nothing to do with the colour of the sky.
  12. Sep 28, 2006 #11
    Water is actually very slightly blue, but you have to have a copious amount of the it to notice this effect. If you just have a glass of water, it will look transparent to you because it is emitting only a tiny amount of "blue light" but when you are looking at an ocean, the effect adds up and you see it as blue.
  13. Sep 29, 2006 #12
    It's perhaps not best described that way, Rozenwyn. You can do a "scattering" experiment with a fishtank, a torch, and a little milk in the water. From the side the beam makes the milky water look bluish. From the end looking up the beam, the milky water looks reddish. So it's kinda of got two different colours depending on how you look at it.

    Colour isn't quite as simple as you might think. Do try the little perception test I mentioned earlier:

    Last edited: Sep 29, 2006
  14. Sep 29, 2006 #13


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  15. Sep 29, 2006 #14


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    Actually, "water is blue because it reflects the color of the sky" is close to being true! You just have to realize that the word "reflects" cannot be taken literally. Water is blue for the same reasons that the sky is blue- blue scatters better than other colors.
  16. Sep 29, 2006 #15
    Have you tried that link?
  17. Sep 29, 2006 #16


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    Yes. I tried it when you first posted it. It's all about visual perception - which has nothing to do with why water is blue.
  18. Sep 29, 2006 #17
    Thanks for doing the perception experiment. As far as I can tell you're the only one who has.

    The point I was trying to make, is that "colour" is more complicated than many people appreciate. A scattering experiment shows (milky) water to appear pale blue side-on to the beam, yet orange/yellow looking up the beam. And yet we know water to be colourless and transparent. Meanwhile the perception experiment shows that one colour can appear to us as two totally different colours.

    That's why "Why is water blue?" is a hard question to answer. The question itself contains wrong assumptions.

    About your last point. All physicist know that a photon has a property that is a frequency, not colour. So surely, don't they all agree that the colour of anything is a perceptual thing?
  19. Sep 29, 2006 #18


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    Water preferentially absorbs certain frequencies of light. This can be demonstrated by any means you see fit, whether involving unreliable human perception or reliable photodetection machinery.
  20. Sep 29, 2006 #19
    I tried it. But I have to say that I wan't that convinced by the first two illusions. For the first one, I was still seeing blue on the left and yellow on the right even after I used the mask. The second one was alright but at first I didn't know if it is my eyes or is it something to do with those yellow and blue layers on top.

    I loved the third illusion though. It definitely convinced me that color can be a matter of perception.
  21. Sep 30, 2006 #20
    Swapnil: thanks. The first one is better if you get a bit of paper, and make a little hole in it to serve as a do-it-yourself mask. You could replicate this in a cliff-top-room experiment involving a projector and a hole in the wall, and then you'd be telling me the sea is yellow.

    Dave: Are we talking at cross purposes here? What does your photodetection machinery actually measure? What colour is a gamma photon? What might a super-evolved bat see? No problem with water absorbing longer frequencies first. But you're the diver, you must have looked sideways two metres down, or done some underwater photography and know there's lots of effects going on:


    All I'm saying is colour is a complicated subject.
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