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Is water blue?

  1. Apr 25, 2009 #1

    DaveC426913

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    I feel silly asking this question because I thought I knew all about this.

    A friend of mine told me that water is actually blue. i.e. its blueness is not merely a matter of preferential scattering (the usual explanation), but that the chemical that is water is actually (very faintly) blue. He said that the colour comes from the interactions of light with the atomic (or was it molecular?) bonds. He also said that water is virtually unique in this regard, all other substances that have colour do not have it this property.

    This is very much against my understanding but I have a lot of faith of him.
     
  2. jcsd
  3. Apr 25, 2009 #2

    mgb_phys

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  4. Apr 26, 2009 #3
    An informative article mgb. On the other hand, what would the nice picture of a lake, at the top of the website look like, reflecting an orange sky? Gray-brown, I think.
     
  5. Apr 26, 2009 #4

    atyy

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    http://www.timelysnow.com/2007/05/11/so-i-guess-qing-%E9%9D%92-really-is-blue/ [Broken]
     
    Last edited by a moderator: May 4, 2017
  6. Apr 26, 2009 #5

    Ivan Seeking

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    Interesting! And might I say, I knew it!!! All of these years I was told otherwise, but while on one our trips to Hawaii, I had convinced myself that the color of the water could not be a result of scattered light only. In fact there must be something about the water in the middle of the pacific or around the islands that [at times?] makes it even bluer than normal.

    This has really bugged me for a long time.

    The other thing that I've noticed is the water from our well system. The water in our area is pretty bad, so I put in a premium filtration system. When the system is operating at peak performance, which is most of the time, and we fill the bathtub, the water is obviously blue. It never looked blue before we put in the new system.
     
    Last edited: Apr 26, 2009
  7. Apr 26, 2009 #6

    atyy

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  8. Apr 26, 2009 #7
    Are there any scuba divers in the house? With increasing depth, Red colored materials appears less red.
     
  9. Apr 26, 2009 #8

    DaveC426913

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    I am a diver.

    There is no question that lots of water appears blue. The question is whether this colour is an intrinsic property of the material or an effect of scattering.

    As a counter-example: the sky appears blue too, but it is not because air is blue.
     
  10. Apr 26, 2009 #9
    Doh! I missed that. I'm not sure I'd use the word intrinsic, though. The distinctions are absorption-->heat vs. scattering or absorption-->reemission, no?
     
  11. Apr 27, 2009 #10

    Ivan Seeking

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    I was always told that the water was simply reflecting the scattered light from the "blue sky". Dave, you are suggesting the scattering of blue light within the water itself?
     
  12. Apr 27, 2009 #11
    I've heard this before too and I think that water appears blue because of both of the ideas mentioned, i.e. it appears strongly blue on a clear day with clear water because it is very slightly blue itself and from preferential scattering.

    Phrasing it as 'water is clear with a blue tinge' would probably be better than 'water is very slightly blue' may be better. I haven't followed up the idea fully though as I had (and still have) other things in my main trains of thought :)
     
  13. Apr 27, 2009 #12

    DaveC426913

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    Well, there's no question that, at depth, everything is very blue. It's certainly not an issue of reflecting the sky.


    Read the article mgb_phys gave in post #2. It is quite informative.

    In particular, note the empirical results in the photo below the graphs. Regardless of what explanation you like, it will have to explain that.


    In a nutshell, the article explains that - while virtually ever other colour seen in nature is the result of photon-electron interaction - the colour of water is virtually unique in that it is the result of molecular vibration stemmnig from the preponderance of OH bonds.
     
    Last edited: Apr 27, 2009
  14. Apr 27, 2009 #13
    Its the same as glass, its inherently transparent looking through it, but if you turned it around and looked through the thick of it, it appears green.

    Anyway cant we just chuck some water in a spectrometer once and for all to finalise this?
     
  15. Apr 27, 2009 #14

    russ_watters

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    Using the word "scattering" as the centerpiece doesn't frame the issue correctly, since water (and any blue object) also "scatters" blue light. That's what diffuse reflection is. Ie, water most certainly is blue because it "scatters" blue light, but that's only half the answer. The other half is that it absorbs other wavelengths. The difference then iis that the water absorbs the other wavelengths and the air does not.
     
  16. Apr 27, 2009 #15

    russ_watters

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    How does one acquire an orange sky? On a cloudy day, the sky is white/gray and the water is still blue (just darker).
     
  17. Apr 27, 2009 #16

    fluidistic

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  18. Apr 27, 2009 #17

    DaveC426913

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    We're not just talking about looking at water from the surface though. Looking through water at-depth is also blue, but has nothing to do with reflection from the sky.
     
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