Why are the sea and lakes blue ? Poll

[Hallu]

I noticed than not many people know why the sea and lakes are blue, why the lagoons have a splendid turquoise color etc... So just a poll to see what you guys think. Please no wikipedia or google before answering, or no guessing, just be honest about what you thought. I myself never knew the answer until my masters degree in physics... And a couple of friends who both have a Ph.D in chemistry didn't know the answer and didn't even believe me when I told them...

[Strider]

It is a reflection of the sky

[Bluegum Mic]

I can't see the poll as Im on my phone but I always though it had to do with the way with which the light is scattered by the water and what particles are suspended in it. Like when you look at some of the glacial lakes they are that amazing milky blue due to the sediment that's in the water from the glacier run off etc.

[jackhinde]

i'd imagine all of the above depending on the locality etc. but i am just a lowly high schoool physics teacher with out the masters...

[nihil]

I think it's about the density of particles. Think about a sunset in an otherwise blue sky. The sky has very few particles comparatively, but cloud is denser, and when the sun sets it sines through many clouds along the angle of the curving earth - clouds stacked up agains one another as it were.

If you talk a fish bowl and put some milk in it and shine a torch through it, as you as more milk it will go from blue to red to brown to black. If you hold a torch by the globe end your hand becomes orange because of the density of molecules in your skin.

So the denser the concentration of molecules, the harder it is for light to penetrate, and just like with bass traveling through walls where trebble is stopped short, longer waves of light penetrate farther, and red light is a longer wave. That's why when a star goes into 'red shift' it means it is traveling away from the observer thus elongating the wave of light over time - or something like that.

So ultraviolet is 'ultra' because it means 'above' in Latin, above violet which is the highest frequency we can see, and 'infra' means below in Latin, hence infrared, below red, the slowest, I.e. longest wave we can see.

So as waves get longer they shift from blue to red in our perceptual system, and dense mediums like cloud or skin filter out the shorter waves.

Don't ask me why it doesn't involve the entire spectrum of color and goes from blue to red, seemingly skipping the middle.

[phan_TOM]

You were obviously studying the wrong degree! any undergraduate in an applied/environmental science degree knows the answer

I think it's worth pointing out that your question also has two different answers, one relating to the blue sky/ocean/lakes and the other to the splendid turquoise colour in some rivers and lakes.

A search on Rayleigh scattering will give you the reason for blue sky and ocean, light is scattered by particles in the air (or water) and the different wavelengths are scattered at varying quantities depending on the amount/type of particles in the atmosphere. Short (blue) wavelengths are scattered more easily which makes the sky appear blue.

Some of those amazing blue/turquoise lakes and rivers are caused by suspended glacial flour, tiny particles of rock crushed under the immense weight of a glacier which become suspended in the water column, making the water look milky or sometimes that amazing blue/green colour.

Not the best explainations but rayleigh scattering and glacial flour are the main causes of the colour we see in the air/water

Don't ask me why it doesn't involve the entire spectrum of color and goes from blue to red, seemingly skipping the middle.

I'm not too sure about your explanation but I think you'll find that it doesn't skip any of the spectrum, a blue day goes into a yellow sunset, then orange & red all depending on how much atmosphere the light has to travel through. I can't remember seeing a green sunset though?

[Son of a Beach]

I disagree with the premise. The sea and the lakes are NOT blue. At least not always. If there is no blue sky, they are not blue (perhaps with some exceptions). During a storm, they are dark grey. Just take a look at your bushwalking photos of lakes during the rain.

Any ripples or waves causes the reflection (AND transmission) to be diffuse and therefore fairly uniform in colour, averaging out the colour of the sky (for open water) or the forrest, mountain, lake bed, or whatever it is that is contributing most to the colour for the angle at which your are viewing it.

WHEN they do look blue, it is primarily due to reflection of a mostly clear sky and this is easy to demonstrate in photos (see below).

There may well be other factors that contribute to their colour, but in general these other factors make it some colour other than blue, or a less-pure blue (eg turquoise).

Other factors include the colour of what is in the water - eg muddy water will look browner. Also the colour of the lake/sea bed if shallow enough and the angle/light is not producing significant reflections off the surface. The turquoise colour is a mixture of the blue light from the sky and the the yellowish sand on the sea bed.

Therefore, I say that IF the water is blue, it is due to reflection of a blue sky (in most cases). If it is is some other colour, then it is due to either reflection of something other than a blue sky, or what is in or under the water.

If I'm wrong, so be it. But here's my photographic evidence.

In front of my house the blue sky is reflected in blue(ish) water - in this case there is a lot of turbidity stirring up mud, making it a bit of a muddy blue colour

Same place, less blue sky, less blue water

Same place again, even less blue sky, even less blue water

Same place yet again, grey sky, grey water

Out bush now... Perfectly blue sky, but the water is ONLY blue where it reflects the sky and it is green/brown where it reflects tree/rocks. Furthermore, it is the colour of the lake bed where it is shallow and the angle and intensity of reflected light is not producing any significant reflection in the foreground

Out bush again. No blue or grey or sky colour in the water, just the colour of the nearby forest and mountain

Here there is only a tiny patch of blue water. It is very clearly a reflection of blue sky, surrounded by other colours, also very clearly reflected by clouds, and parts of the nearby landscape

[Hallu]

Well there was bound to be some semantic debate with such a simple question. But think about this : if the sky is the reason, why is an indoor swimming pool with a white bottom, white light and white ceiling still blue ?

[phan_TOM]

I disagree with the premise. The sea and the lakes are NOT blue

and I must refute this spurious claim! Water IS blue. Yes it does reflect the blue sky (or a cloudy or a sunset sky which as you correctly stated can make it appear grey, silver or bright red!) and it can also be coloured by whatever dissolved particles are suspended in it, but... I remember hearing Dr Karl on the radio a short time ago and he was saying that water is actually blue & I aint gonna argue with Dr Karl

I searched for Dr Karl and the colour of water and found this excellent video which should help clear up any doubts that you may have

http://www.abc.net.au/science/video/2012/08/10/3565196.htm

[Rob A]

Be my guess ...

revised-graph-curves.gif (13.48 KiB) Viewed 18828 times

But that doesnt actually descrbe the reason why. So after looking, so I cant vote, bouncy solution, unique and yes Blue for a clean column of water. Presumably this is the same mechanism as bird feather and insect wing non pigment colour outside our visible.

[doogs]

Absorption and refraction of the sunlight (or artificial light!). The colour can be affected by suspended particles such as fine clays in glacial lakes.

[Hallu]

Refraction is just the deflection of a ray of light or a ray of sound when it goes through the interface between 2 different media. You must be thinking about scattering which is a different phenomenon. Hard sometimes to remember the difference between refraction, reflection, scattering, dispersion, absorption, transmission, evanescence etc...

[Son of a Beach]

Well there was bound to be some semantic debate with such a simple question. But think about this : if the sky is the reason, why is an indoor swimming pool with a white bottom, white light and white ceiling still blue ?

I don't think I've ever seen a swimming pool that is not painted blue - the vast majority are. And for very good reason. But...

I thought we were talking about why the (outside) sea and lakes are blue, not why indoor swimming pools are blue. Because the answer is completely different.

Anyhow... if the water in a swimming pool is naturally so blue without any reflection from the sky, why do they paint swimming pools blue? Its because any natural blue is so insignificant as to require paint to make it look more blue.

Again, there may be other factors that make water look various colours, including blue, but reflection is by far the most significant. So much so, that the sea and lakes (in the vast majority of cases) do not look even vaguely blue if the sky is not blue.

So if you think that most of the sea/lakes are blue for some reason other than reflecting a blue sky, please explain why (in most cases) they are not blue when there is no blue sky.

[Rob A]

Unknown.jpeg (10.74 KiB) Viewed 18788 times

That its colour can be overidden by numerous factors doesnt negate the fact that water is blue.
My guess is that the original question needs defining.

[Hallu]

Well let's give the answer before it gets more heated. The answer is a mix of all 3.

Yes, the reflection of the blue sky helps seeing the sea/lakes blue. Nevertheless that doesn't explain then why it's not the same blue for lagoons and deep seas, or why swimming pools are blue. By the way many swimming pools have a white bottom, and if you go a few meters to the bottom and look up to the ceiling, still blue.

As many said, water is indeed blue. A very faint blue, which is why your glass of water is transparent. As the depth increases, it gets more and more blue. The reason is that water molecules reflect blue light more than all the other wavelengths (or absorb it less), as shown on this absorption spectrum : http://3.bp.blogspot.com/_fa6AZDCsHnY/S ... rption.gif . You can see that there's quite a small difference between the absorption of blue and the absorption of yellow/red/etc... (infrared and far ultraviolet are heavily absorbed, but not near UV which is why you still need sunscreen during a swim) This small difference explains why you need a great thickness of water to "enhance" it. And that property is simply due to the water molecule itself, nothing else. Each molecule has a specific absorption spectrum, giving it its color in bulk. Use a light having its blue component removed on a pool of water, you're gonna see the water green.

Now scattering on particles in suspension in the water also helps giving it its color. Rock flour for turquoise lakes as many said, through scattering processes, some algae make it green, pink, etc...

But the explanation many don't know is that water is naturally blue. It takes a few meters of water to see it, just go in a white bottom swimming pool, immerse yourself, and look at the bottom, then look at the walls. You'll see it's more blue in the second case because light has traveled a thicker layer of water. You can see the effect here : http://www.youtube.com/watch?v=M9GRiXBS0e0 the first swimmer on screen is lightly blue, the last one is a deeper blue. Still think it's because of the walls or the bottom ? Have you ever seen an underwater documentary where the water isn't blue ?

And here's a picture for SonofaBeach, of a river, taken on a clouded in day. The water near the bank is clear, because shallow, and as you get deeper, it gets blue (of course in this one there's also rock flour giving it this turquoise color).

[Son of a Beach]

That its colour can be overidden by numerous factors doesnt negate the fact that water is blue.
My guess is that the original question needs defining.

I was going to say much the same thing. If the question had been "What colour is water?" that would have been a very different question.

The colour of anything could be defined in different ways. But most people tend to accept that the colour of something is the colour that their eyes see - ie, the predominant colour that reaches their eyes after reflecting from, emanating from, or being transmitted through.

[Son of a Beach]

And here's a picture for SonofaBeach, of a river, taken on a clouded in day. The water near the bank is clear, because shallow, and as you get deeper, it gets blue (of course in this one there's also rock flour giving it this turquoise color).

That picture doesn't look very convincing to me.

I do not doubt that water has a blue colour. But I stand by my earlier statements that the primary colour of the vast majority of outdoor water bodies is that which is reflected from it's surface.

The rest is really just semantics.

[ILUVSWTAS]

But most people tend to accept that the colour of something is the colour that they see

A bit like margarine really being a crappy gray colour, not the yellow it seems?

[Hallu]

No the "vast majority" part is not true, it completely depends on the position of the sun and the observer. You can get full reflection or none at all. Since you took those picture, you know that. Do you see clouds here reflected here : download/file.php?id=20789&mode=view ? How about this picture : download/file.php?id=20790&mode=view , you have a pure water lake on the left, a rock flour lake on the right. Perfect sunny day, you can see some cloud reflections and yet you clearly see they have different colours. Another perfect picture to illustrate the thickness effect : http://www.planetware.com/i/photo/aitut ... cok101.jpg You've got a summary with link to articles here if you're still not convinced : http://en.wikipedia.org/wiki/Color_of_water

[Son of a Beach]

heheh... I'm still far from convinced.

I'd like to see pictures of those same two places during a dark storm. They just proved the point - blue sky, blue water!

The third picture again illustrates what I said earlier - blue sky, blue reflection, shallow water with sandy bottom causes reflections from the sand (yellowish) to mix with the blue from the surface, causing a turquoise.

That wiki article is about the colour of water, NOT about the colour of the sea and lakes. Again, these are two very different things (depending on your point of view). Where it does talk about lakes and the sea, it appears to support my case - that there are several factors, but in most situations the reflection from the surface is the primary factor.

Yes, the position of the observer certainly makes a difference. Being close to the edge of the water is one of the very few places that reflection is not the primary source of light (and hence colour) from the water. That proximity effect is clear in your third image, as in some of the images I posted earlier. But the primary colour is still from reflection anywhere other than close by (although the other colours certainly do have some impact elsewhere).

Apart from that proximity (and then only near the edge), in what situation do you get no reflection from water at all???? Except for wearing polaroids and being very close to the water, the vast majority of light coming from the water is that which is reflected off the top of it. You will not see picture perfect reflections in most cases, but the light is diffused by ripples and waves, giving a more uniform colour. Usually the colour of the sky, blue or grey.

What is the colour of water on a pitch black dark night when there is no light (assuming no phosphorescent critters)?

PS. I'm about to go away bushwalking, so sorry if I don't follow up further on this.

PPS. Sorry for hijacking the topic a bit, and for being so contrary. I just thought it was an opportunity for a bit of a fun debate.

[Wollemi]

Other factors include the colour of what is in the water - eg muddy water will look browner.

Other factors are certainly influenced by urban runoff, construction spoil and disruption of the bonding of natural materials found in nature. Such a photo demonstrates this.

Of course I am envious that you are/were building a new house so close to the waters edge.

[Hallu]

lol you really think yellow from the sand and blue from the sky make turquoise ? If you've got a nice mirror reflection perfectly reflecting the blue of the sky, you could put orange on the bottom and the reflection would still be the same. When you see those shades of dark blue and turquoise, it's because reflection isn't a major part of what you're seeing. It's not that hard : if the sun is on your back, you can't see the rays bouncing off the surface because they're going away from you, it's physics, it's Snell-Descartes law. I suggest you read this : http://en.wikipedia.org/wiki/Snell%27s_law So to answer your question, to see reflection from the surface, you need to be on the path of reflected sun rays. Hence you need the sun and the lake/sea in front of you, or the surface to be curved (waves or ripples).

And nowhere in the wikipedia article does it say that reflection of the sky is the primary factor in most situations. Read it again. It only says "While this reflection contributes to the observed color, it is not the sole reason." and "The surfaces of seas and lakes often reflect blue skylight, making them appear bluER. The relative contribution of reflected skylight and the light scattered back from the depths is strongly dependent on observation angle."

You don't seem to understand something : light actually penetrates the surface, and get bounced back by the water molecules or the molecules in suspension such as rock flour (see the picture with the 2 lakes : if reflection was the main culprit, you wouldn't see such a difference in colour). And unlike reflection on the surface, you can see it from anywhere because the light scattering is done in every direction, it just depends on the orientation of the water molecule in space, which is random. This is how you get those different shades of blue, depending on how much water the light travelled in before being bounced back on the molecules. We can talk in photons if you want, grains of light. In shallow water of let's say 50 cm thick, let's assume 100 photons of each colour are reflected on water molecules in average (the rest being reflected on the bottom or on the surface). It's maybe about 101 blue photons seen by your eye versus 99 red. Hence you saw 2 more blue, not much of a difference, and not enough photons reflected for you to see anything : it's transparent. Now let's have a 1 km deep layer of water. Let's assume about 100 million of each are reflected back to you. Then you're gonna see 101 million of blue against 99 million of red. That's 2 million more blue, the rest is like white light because all the wavelengths mix : you see the water blue.

And no you cannot talk about colour on a pitch black dark night lol. Colour only exists when there's light...

[Rob A]

So if you think that most of the sea/lakes are blue for some reason other than reflecting a blue sky, please explain why (in most cases) they are not blue when there is no blue sky.

Well, they still are. Remember we arent talking about 'perceptions'.

"Blue water is the only known example of a natural color caused by vibrational transitions."
"The blueness of the water is neither due to light scattering (which gives the sky its blue color) nor dissolved impurities "
"Most (other) molecules have vibrational energies that are lower in frequency (longer in wavelength) than that of water, falling in the range of far infrared or thermal vibrations rather than in the visible light range."

Have a squiz here ... Its all pretty easy to follow. http://www.webexhibits.org/causesofcolor/5B.html Feel free to follow through the previous and nexts.

Just because you park your red Ferrari in the garage with the light off, its still red. When you forget the handbrake and drop it off the wharf and it is sitting in thirty feet of water, it might appear green but really, its still red.

Of course this doesnt mean if you fly over the Red River it isnt that for the silt, or the Black River for its deep shadows, or a red lake isnt full of algae.

[Son of a Beach]

lol

Don't laugh at me. You're making me feel all self conscious.

you really think yellow from the sand and blue from the sky make turquoise ? If you've got a nice mirror reflection perfectly reflecting the blue of the sky, you could put orange on the bottom and the reflection would still be the same.

Not true. I've seen plenty of lakes with the blue sky reflected in them, while I can see rocks and fish beyond the blue reflection.

When you see those shades of dark blue and turquoise, it's because reflection isn't a major part of what you're seeing. It's not that hard : if the sun is on your back, you can't see the rays bouncing off the surface because they're going away from you, it's physics, it's Snell-Descartes law. I suggest you read this : http://en.wikipedia.org/wiki/Snell%27s_law So to answer your question, to see reflection from the surface, you need to be on the path of reflected sun rays. Hence you need the sun and the lake/sea in front of you, or the surface to be curved (waves or ripples).

Now I'm not sure if you're serious or just joking. Nowhere have I mentioned reflections from the sun. I don't need to have the sun in front of me to see reflections of the sky. This is blindingly obvious to anybody who's ever used a mirror. I'm talking about reflections of the sky, not the sun.

And nowhere in the wikipedia article does it say that reflection of the sky is the primary factor in most situations. Read it again. It only says "While this reflection contributes to the observed color, it is not the sole reason." and "The surfaces of seas and lakes often reflect blue skylight, making them appear bluER. The relative contribution of reflected skylight and the light scattered back from the depths is strongly dependent on observation angle."

Fair enough. It doesn't say it explicitly. But when talking about all of the contributing factors in the colour of oceans and lakes the very first one it mentions is reflections of the sky. So it implies that it is the primary factor. That's my interpretation, but you are correct it doesn't explicitly state that it is the primary factor.

You don't seem to understand something : light actually penetrates the surface, and get bounced back by the water molecules or the molecules in suspension such as rock flour (see the picture with the 2 lakes : if reflection was the main culprit, you wouldn't see such a difference in colour).

Why would you think I don't understand that, when I've explicitly stated that is the case. I've mentioned mud in turbid water, as I've mentioned the lake/sea bed reflecting light. You may need to go back and read what I actually did say again.

If reflection was not the main culprit, then why are the sea and lakes blue when the sky is blue, and not even vaguely blue looking (in MOST places) when there are only dark clouds?

And unlike reflection on the surface, you can see it from anywhere because the light scattering is done in every direction, it just depends on the orientation of the water molecule in space, which is random. This is how you get those different shades of blue, depending on how much water the light travelled in before being bounced back on the molecules. We can talk in photons if you want, grains of light. In shallow water of let's say 50 cm thick, let's assume 100 photons of each colour are reflected on water molecules in average (the rest being reflected on the bottom or on the surface). It's maybe about 101 blue photons seen by your eye versus 99 red. Hence you saw 2 more blue, not much of a difference, and not enough photons reflected for you to see anything : it's transparent. Now let's have a 1 km deep layer of water. Let's assume about 100 million of each are reflected back to you. Then you're gonna see 101 million of blue against 99 million of red. That's 2 million more blue, the rest is like white light because all the wavelengths mix : you see the water blue.

And no you cannot talk about colour on a pitch black dark night lol. Colour only exists when there's light...

So its seems that we agree on 99.9% here. The only difference being how significant each factor is in determining the colour that we actually perceive. I say that reflection accounts for the vast majority of colour in the vast majority of cases. Clearly there are some exceptions, as I stated right from the start.

[Son of a Beach]

So if you think that most of the sea/lakes are blue for some reason other than reflecting a blue sky, please explain why (in most cases) they are not blue when there is no blue sky.

Well, they still are. Remember we arent talking about 'perceptions'.

Well, I disagree. Maybe it's our idea of what a "colour" is that is where the disagreement comes from. Colours are all about perceptions. Yes, yes, they're also about what frequency of light they reflect, or transmit, or emit. But in the end, it's what we perceive that makes it a colour to us.

But again, this is just semantics and beside the point of the original question. The original question was not "what colour is water".

I believe that the intent of the original question was about perceptions (ie, why do the lakes and see look blue). If that was not the case, the the question was not explicit enough.

I gotta go now. Bushwalking in the morning.

[nihil]

Nothing has a color, it's the absorption of the light spectrum (or non absorption in the case of refraction) so black reflects all the spectrum, white, none.

The instances of color other than blue in Son of a beach's photos are a function of the quality of light hitting the scene, which is always a function of the cloud cover and the atmosphere in general. An overcast day renders colours grey because the clouds have defused the light and all the colours are bouncing off of each other. It's also a function of how much saturation was applied to the digital raw information of the picture.

[Rob A]

Nothing has a color, it's the absorption of the light spectrum (or non absorption in the case of refraction) so black reflects all the spectrum, white, none.

The instances of color other than blue in Son of a beach's photos are a function of the quality of light hitting the scene, which is always a function of the cloud cover and the atmosphere in general. An overcast day renders colours grey because the clouds have defused the light and all the colours are bouncing off of each other. It's also a function of how much saturation was applied to the digital raw information of the picture.

Um ahh. Someones rewriting the laws of physics. Move over Max Karl.

[stepbystep]

There is another radical school of thought that revolves around the perceptions of mass consciousness.
Simply stated we see what our ancestors over thousands of generations, 'scientists' or not have trained us to believe we see and really each one of us are seeing something different but our brains are trained to see what we are told we should be seeing.

So there!

[Hallu]

You're trying to turn scientific facts into philosophical questions or to blame the question to twist it a bit your way. This is where it gets quite not interesting any more. The point of my question was to see what most people think (that's why there's a poll in it). The majority of people either think it's only because of the reflection of the sky, or know this can't be the full reason but don't know what it is. And many people, when explained that water is blue, and that it explains the deep blue or light blue depending on depth, still remain sceptical. The problem is it involves quite a great deal of physics and chemistry, and it can get pretty confusing.

I'm not going to try to convince you again, I think you should simply do it yourself by looking at lakes and other bodies of water.

[stepbystep]

You're trying to turn scientific facts into philosophical questions or to blame the question to twist it a bit your way. This is where it gets quite not interesting any more. The point of my question was to see what most people think (that's why there's a poll in it). The majority of people either think it's only because of the reflection of the sky, or know this can't be the full reason but don't know what it is. And many people, when explained that water is blue, and that it explains the deep blue or light blue depending on depth, still remain sceptical. The problem is it involves quite a great deal of physics and chemistry, and it can get pretty confusing.

I'm not going to try to convince you again, I think you should simply do it yourself by looking at lakes and other bodies of water.

Bah, the concept of scientific 'facts' is a human construct to give order to all things. I much prefer the 'muddied waters' of philosophical thought and creative thinking(like religions, fascinating stuff).