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Thread: Why is the sea blue?

  1. #1 Why is the sea blue? 
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    I've come across several answers to this question.

    Some say it's simply because it reflects the sky. Others say it's because other visible light is more readily absorbed (but I thought blue light was absorbed more than other visible light) and yet others say it's got something to do with Raman scattering (given it's so weak can it really be due to the Raman effect?). In each case the details seem vague. Does anyone know of any good resources addressing this question, or explain it to me?


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    Quote Originally Posted by epimetheus View Post
    Does anyone know of any good resources addressing this question, or explain it to me?
    What color is water? | Causes of Color


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    Quote Originally Posted by epimetheus View Post
    I've come across several answers to this question.

    Some say it's simply because it reflects the sky. Others say it's because other visible light is more readily absorbed (but I thought blue light was absorbed more than other visible light) and yet others say it's got something to do with Raman scattering (given it's so weak can it really be due to the Raman effect?). In each case the details seem vague. Does anyone know of any good resources addressing this question, or explain it to me?
    Well surely this is due to Rayleigh scattering, is it not? That is why the sky is blue. Not absorption, but scattering. Blue, having a shorter wavelength, is scattered more than the other colours.

    The sea is not blue, by the way, on a grey day in the West of Scotland or Ireland. James Joyce vividly described the "snot-green, scrotum-tightening sea". I remember it so well from my childhood holidays! My guess is it is all to do with suspended fine particles and the Rayleigh scattering from them.

    I have never heard Raman scattering mentioned in this context and would find it very doubtful indeed, due to the low intensity of Raman processes that we were just discussing on your other thread. Can you provide a reference to a source talking about Raman scattering in this context? I'd be intrigued to read it.
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    Quote Originally Posted by epimetheus View Post
    I've come across several answers to this question.

    Some say it's simply because it reflects the sky. Others say it's because other visible light is more readily absorbed (but I thought blue light was absorbed more than other visible light) and yet others say it's got something to do with Raman scattering (given it's so weak can it really be due to the Raman effect?). In each case the details seem vague. Does anyone know of any good resources addressing this question, or explain it to me?
    Raman scattering is basically scattering of light with a change in frequency. Rayleigh scattering explains how light scatters off of atoms. But what about molecules?

    "It is also possible for the incident photons to interact with molecules in such a way that energy is either gained or lost so that scattered photons are shifted in frequency. Such inelastic scattering is called Raman scattering."

    Raman Scattering

    Theory of Raman Scattering - B&W Tek
    Last edited by Secular Sanity; February 9th, 2018 at 10:35 AM.
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    Quote Originally Posted by Secular Sanity View Post
    Quote Originally Posted by epimetheus View Post
    I've come across several answers to this question.

    Some say it's simply because it reflects the sky. Others say it's because other visible light is more readily absorbed (but I thought blue light was absorbed more than other visible light) and yet others say it's got something to do with Raman scattering (given it's so weak can it really be due to the Raman effect?). In each case the details seem vague. Does anyone know of any good resources addressing this question, or explain it to me?
    Raman scattering is basically scattering of light with a change in frequency. Rayleigh scattering explains how light scatters off of atoms. But what about molecules?

    "It is also possible for the incident photons to interact with molecules in such a way that energy is either gained or lost so that scattered photons are shifted in frequency. Such inelastic scattering is called Raman scattering."

    Raman Scattering

    Theory of Raman Scattering - B&W Tek
    Yes, we've just been discussing all that in some detail on another thread here.
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    You might want to ask yourself why a "chemist" (even an old retired one) would not know the answer to this question.

    Quote Originally Posted by exchemist View Post
    Well surely this is due to Rayleigh scattering, is it not? That is why the sky is blue. Not absorption, but scattering. Blue, having a shorter wavelength, is scattered more than the other colours.

    The sea is not blue, by the way, on a grey day in the West of Scotland or Ireland. James Joyce vividly described the "snot-green, scrotum-tightening sea". I remember it so well from my childhood holidays! My guess is it is all to do with suspended fine particles and the Rayleigh scattering from them.
    Quote Originally Posted by epimetheus View Post
    I've come across several answers to this question.

    Some say it's simply because it reflects the sky. Others say it's because other visible light is more readily absorbed (but I thought blue light was absorbed more than other visible light).

    The blue wavelengths of light are scattered, similar to the scattering of blue light in the sky but absorption is a much larger factor than scattering for the clear ocean water. In water, absorption is strong in the red and weak in the blue and so red light is absorbed quickly in the ocean leaving blue. Almost all sunlight that enters the ocean is absorbed, except very close to the coast.

    https://en.wikipedia.org/wiki/Ocean_color
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    Water is essentially transparent in the visible region of the spectrum.

    There is vibrational absorption in the IR and electronic absorption in the UV. Nothing in the visible, except conceivably a very tiny contribution from vibrational overtones, I suppose.

    Brief summary here: http://hyperphysics.phy-astr.gsu.edu...al/watabs.html

    Any significant visible region absorption by seawater must, I should have thought, either be due to phytoplankton or other absorbing entities suspended in it, or be really scattering, i.e. not absorption, by particles. That would be Rayleigh scattering.
    Last edited by exchemist; February 9th, 2018 at 01:46 PM.
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    Quote Originally Posted by exchemist View Post
    Water is essentially transparent in the visible region of the spectrum.

    There is vibrational absorption in the IR and electronic absorption in the UV. Nothing in the visible, except conceivably a very tiny contribution from vibrational overtones, I suppose.

    Brief summary here: Transparency of Water in the Visible Range

    Any significant visible region absorption by seawater must, I should have thought, either be due to phytoplankton or other absorbing entities suspended in it, or be really scattering, i.e. not absorption, by particles. That would be Rayleigh scattering.
    Well, as usual, you thought wrong.

    Water is intrinsically blue.

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

    Water owes its blueness to selective absorption in the red portion of its visible spectrum. The absorbed photons promote transitions to high overtone and combination states of nuclear motions of the molecule, i.e. to highly excited vibrations.
    Colors from vibrations | Causes of Color
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    Quote Originally Posted by Secular Sanity View Post
    Quote Originally Posted by exchemist View Post
    Water is essentially transparent in the visible region of the spectrum.

    There is vibrational absorption in the IR and electronic absorption in the UV. Nothing in the visible, except conceivably a very tiny contribution from vibrational overtones, I suppose.

    Brief summary here: Transparency of Water in the Visible Range

    Any significant visible region absorption by seawater must, I should have thought, either be due to phytoplankton or other absorbing entities suspended in it, or be really scattering, i.e. not absorption, by particles. That would be Rayleigh scattering.
    Well, as usual, you thought wrong.

    Water is intrinsically blue.

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

    Water owes its blueness to selective absorption in the red portion of its visible spectrum. The absorbed photons promote transitions to high overtone and combination states of nuclear motions of the molecule, i.e. to highly excited vibrations.
    Colors from vibrations | Causes of Color
    Well thanks for this - seems the contribution from vibrational overtones was more relevant than I had thought.

    Interesting - learn something new every day.
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    Quote Originally Posted by exchemist View Post
    Well thanks for this - seems the contribution from vibrational overtones was more relevant than I had thought.

    Interesting - learn something new every day.
    You’re welcome.
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    After some further thought, I suppose it is just worth making the point that this blueness, or blue-greenness of long path lengths of water - due, so I learn, to very slight absorption from vibrational overtones - has nothing to do with why the sea appears blue to a watcher from a ship or the shore. The blueness in that case is reflected light from the sky.
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    Quote Originally Posted by exchemist View Post
    After some further thought, I suppose it is just worth making the point that this blueness, or blue-greenness of long path lengths of water - due, so I learn, to very slight absorption from vibrational overtones - has nothing to do with why the sea appears blue to a watcher from a ship or the shore. The blueness in that case is reflected light from the sky.
    No. Have you ever been on a ship? Even on a cloudy day the ocean will still appear blue.

    "When sunlight hits the ocean, some of the light is reflected back directly, but most of it penetrates the ocean surface and interacts with the water molecules that it encounters. The red, orange, yellow, and green wavelengths of light are absorbed and so the remaining light we see is composed of the shorter wavelength blues and violets."

    This is the main reason why the sea is blue.

    https://en.wikipedia.org/wiki/Ocean_color
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    Quote Originally Posted by epimetheus View Post
    I've come across several answers to this question.

    Some say it's simply because it reflects the sky. Others say it's because other visible light is more readily absorbed (but I thought blue light was absorbed more than other visible light) and yet others say it's got something to do with Raman scattering (given it's so weak can it really be due to the Raman effect?). In each case the details seem vague. Does anyone know of any good resources addressing this question, or explain it to me?
    It's a reflection of the sky.
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    According to this blog, and a few others I can't locate now, it is due to Raman scattering. However, I think the preferential absorption of other visible wavelengths, maybe with a small component of reflecting the sky, makes sense.

    I think the misconception comes from how the Raman effect was discovered. Raman describes how wondering why the Mediterranean sea is so blue led to a series of experiments which ultimately led to the discovery of inelastic scattering. But this doesn't mean the sea is blue due to Raman scattering, only that wondering why the sea is blue led to the discovery of Raman scattering.
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    Quote Originally Posted by epimetheus View Post
    According to this blog, and a few others I can't locate now, it is due to Raman scattering. However, I think the preferential absorption of other visible wavelengths, maybe with a small component of reflecting the sky, makes sense.

    I think the misconception comes from how the Raman effect was discovered. Raman describes how wondering why the Mediterranean sea is so blue led to a series of experiments which ultimately led to the discovery of inelastic scattering. But this doesn't mean the sea is blue due to Raman scattering, only that wondering why the sea is blue led to the discovery of Raman scattering.
    Exactly. Thumbs up, epimetheus!
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    Quote Originally Posted by Secular Sanity View Post
    Quote Originally Posted by exchemist View Post
    After some further thought, I suppose it is just worth making the point that this blueness, or blue-greenness of long path lengths of water - due, so I learn, to very slight absorption from vibrational overtones - has nothing to do with why the sea appears blue to a watcher from a ship or the shore. The blueness in that case is reflected light from the sky.
    No. Have you ever been on a ship? Even on a cloudy day the ocean will still appear blue.

    "When sunlight hits the ocean, some of the light is reflected back directly, but most of it penetrates the ocean surface and interacts with the water molecules that it encounters. The red, orange, yellow, and green wavelengths of light are absorbed and so the remaining light we see is composed of the shorter wavelength blues and violets."

    This is the main reason why the sea is blue.

    https://en.wikipedia.org/wiki/Ocean_color
    This is untrue. The sea most certainly does not appear blue when seen from ship or shore on an overcast day (such as I am very familiar with from years of steamer trips, walking and and sailing in the West of Scotland and Brittany.) Under an overcast sky the sea appears grey when viewed obliquely, as from the deck of a ship or the shore, or green if one peers down vertically into the depths - hence Joyce's "snot-green, scrotum-tightening sea", which I mentioned earlier. (Joyce and I evidently agree it is often not blue in Ireland either, as I can testify from childhood holidays near Cork.)
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    Quote Originally Posted by exchemist View Post
    This is untrue.
    No it's not. Where I live there are grey skies and deep blue seas.

    Quote Originally Posted by exchemist
    The sea most certainly does not appear blue when seen from ship or shore on an overcast day (such as I am very familiar with from years of steamer trips, walking and and sailing in the West of Scotland and Brittany.) Under an overcast sky the sea appears grey when viewed obliquely, as from the deck of a ship or the shore, or green if one peers down vertically into the depths - hence Joyce's "snot-green, scrotum-tightening sea", which I mentioned earlier. (Joyce and I evidently agree it is often not blue in Ireland either, as I can testify from childhood holidays near Cork.)
    On an overcast day, it's not grey due to the color of the sky. It's about what's in the water.

    BBC - Earth - Is the sea really blue?

    You should not need me to tell you this. There are numerous places to find it, starting with Wikipedia.

    If you are really this helpless and dependent, I begin to wonder if you are really suited for this type of forum.

    *SWOOSH* Point scoring c__t scores again. Maybe you should sit this one out, old man.
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    You feel that being rude and insulting others is acceptable behavior why?
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    Secular Sanity be civil or be gone.

    I too spent years on the ocean as a New England gillnetter and never saw a deep blue ocean under grey skies. If you have an example, rather than being a rude Ahole and suggesting someone shouldn't be here, post a picture or present a science-based argument for conditions that might cause that effect.
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    Quote Originally Posted by Paleoichneum View Post
    You feel that being rude and insulting others is acceptable behavior why?
    I think that calling someone a point scoring cunt is more insulting and unacceptable, don't you?

    Quote Originally Posted by Lynx_Fox View Post
    Secular Sanity be civil or be gone.

    I too spent years on the ocean as a New England gillnetter and never saw a deep blue ocean under grey skies.

    If you have an example, rather than being a rude Ahole and suggesting someone shouldn't be here, post a picture or present a science-based argument for conditions that might cause that effect.
    A rude ahole? I simply mimicked his behavior.

    Quote Originally Posted by exchemist View Post
    You should not need me to tell you the form of a wave equation. There are numerous places to find this, starting with Wikipedia.

    If you are really this helpless and dependent, I begin to wonder if you are suited to this type of study.
    I already presented the science behind it. It’s not the clouds that make the water appear grey and cloudy. The water is richer in nutrients.

    https://science.nasa.gov/earth-science/oceanography/living-ocean/ocean-color
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    A satellite picture, which is what your article presents, is not the same as what your eyes will see if you are on shore or in a boat because the angle is incidence makes what you see almost entirely reflective.

    Perhaps before your launch into insults, you should review the conservation since exchemist perspective from a boat was very clear.

    The only one I see tossing insults is you Secular and you're the only one who used that particular "point scoring .... term, which was way over the top. Last warning before you get a couple days off.
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    Quote Originally Posted by Lynx_Fox View Post
    A satellite picture, which is what your article presents, is not the same as what your eyes will see if you are on shore or in a boat because the angle is incidence makes what you see almost entirely reflective.

    Perhaps before your launch into insults, you should review the conservation since exchemist perspective from a boat was very clear.

    The only one I see tossing insults is you Secular and you're the only one who used that particular "point scoring .... term, which was way over the top. Last warning before you get a couple days off.
    Well, I didn’t come up with point scoring term on my own. I had a little help.

    The Speed of Light Paradox

    Did you read the BBC article that I linked?

    "Our own experiences suggest the colour of the sea and oceans can change markedly according to both time and place, from brilliant turquoises and whitish greens, through ultramarine, navy blues, to dishwater greys and mucky browns.

    The variation in sea colour, it turns out, is due to both physics and biology.

    BBC - Earth - Is the sea really blue?
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    Well now we are getting somewhere. These links acknowledge the wide variation is the colour of the sea, under different circumstances. They also acknowledge the role of phytoplankton, I see - and even refer to part of the same Joyce quotation, though not the scrotum-tightening bit.

    The picture of the sea on the BBC link in particular illustrates my point that it often looks grey, especially around the shores of my native country.

    As for who should be here on the forum, that is obviously up to the Mods. I am happy to stand on my track record here. If anyone wants to refer me to the Mods to be chucked out for incompetence, or bad science (as has been alleged), I am content for them to do so.

    It is true I did once describe SS as a "point-scoring c--t" but quite frankly, looking at this exchange, I think this was no more than a simple statement of fact.
    Last edited by exchemist; February 19th, 2018 at 04:46 AM.
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    Forget the sea: why is this thread so blue?
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  26. #25  
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    Secular, on of the things I learned early in life:

    "Monkey see, monkey do" isnt an acceptable excuse.
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    Quote Originally Posted by epimetheus View Post
    Forget the sea: why is this thread so blue?
    Well if you trace your way through the posts I think you will be able to spot where a sour note enters the discussion.

    But the good thing about it that the answer is really quite complex and I at least have learnt something new, viz. that vibrational overtone absorption is partly responsible for the blue appearance light emanating from a long column of water or ice, in addition to the more obvious scattering effects.

    So thank you very much for the question.
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    Quote Originally Posted by exchemist View Post

    As for who should be here on the forum, that is obviously up to the Mods. I am happy to stand on my track record here. If anyone wants to refer me to the Mods to be chucked out for incompetence, or bad science (as has been alleged), I am content for them to do so.
    I think you’re missing the point. You were merely hoisted by your own petard. All I did was replace the word "study" with "forum".

    Quote Originally Posted by exchemist View Post
    You should not need me to tell you the form of a wave equation. There are numerous places to find this, starting with Wikipedia.

    If you are really this helpless and dependent, I begin to wonder if you are suited to this type of study.
    Quote Originally Posted by exchemist
    I prefer to debate these issue in such a way that the participants feel encouraged, rather than discouraged, in continuing.
    Yeah, right. I think you’re condescending, arrogant, and take pleasure in making others feel inferior.

    Quote Originally Posted by exchemist
    It is true I did once describe SS as a "point-scoring c--t" but quite frankly, looking at this exchange, I think this was no more than a simple statement of fact.
    Self-justification.

    https://en.wikipedia.org/wiki/Self-justification
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    By the way, I just went for a drive along the Upper Niagara River Parkway. Weather today cloudy, 6°C, no wind, slight rain, water moving but somewhat flat, Sun would be at it approximately noon hour height albeit behind clouds and there's a slight green tint to the river today. Granted the river runs over a primarily limestone subsurface. Should I be seeing some other colour?
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    Quote Originally Posted by zinjanthropos View Post
    Should I be seeing some other colour?
    Nope, and you’re right, Zinman.

    "The startling green colour of the Niagara River is a visible tribute to the erosive power of water. An estimated 60 tons of dissolved minerals are swept over Niagara Falls every minute. The colour comes from the dissolved salts and “rock flour,” very finely ground rock, picked up primarily from the limestone bed but probably also from the shales and sandstones under the limestone cap at the falls."

    https://www.niagaraparks.com/visit-n...facts-figures/
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    The red, orange, yellow, and green wavelengths of light are absorbed so that the remaining light seen is composed of the shorter wavelength blues and violets. This is the main reason why the sea's color is blue. Some constituents of sea water can influence the shade of blue of the ocean....
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    So i wanted to add this picture. The color of the sea is a combination of absorbance, fluorescence, color of the seafloor (if undeep), and some scattering effects.
    Growing up, i marveled at star-trek's science, and ignored the perfect society. Now, i try to ignore their science, and marvel at the society.

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    There are several theories:
    Blue wavelengths are absorbed the least by the deep sea water and are scattered and reflected back to the observer’s eye
    Particles in the water may help to reflect blue light
    The sea reflects the blue sky....
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    Quote Originally Posted by Patricia Riffle View Post
    There are several theories:
    Blue wavelengths are absorbed the least by the deep sea water and are scattered and reflected back to the observer’s eye
    Particles in the water may help to reflect blue light
    The sea reflects the blue sky....
    All these points have already been extensively debated, earlier in this thread.
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