Why water is blue....
Doug Shakel
this. I think the correct answer is on the order of "blue is the color of
water". The trouble is, we are all usually very personally familiar with
water every day, we are very aware of the hazards associated with water that
isn't clear and colorless, and so if it's obviously blue, but know that's
supposed to be ok, we'd like an explanation.
When I was a teen-ager, I worked as a life guard / pool caretaker at an Army
base. Their pool was large, deep end about 10 feet, and every spring painted
bright white! As the pool was filled, the blue color would become evident as
the water was about 4 to 5 feet deep in the deep end. By the time the whole
pool was filled, all the pool appeared blue except in the 3 to 4 foot deep
shallows.
When you visit areas like Pensacola, where the sand is blindingly white, the
water is VERY blue in appearance.
I don't buy sky reflections either. Go out on a really grey day and lower a
white object in to clear clean ocean or lake water. It starts to look blue
after it gets down 5 feet or so.
As to "light scattering"... Well, in a sense that COULD be correct, but then
that just begs the question of "why BLUE" instead of another color.
Atmospheric scattering for blue skies (which itself takes a few knocks every
couple of years or so) is usually ascribed to particulate content.
But it's hard to blame particles in really clear, clean water.
I'm also leary about the "Tyndall effect". Jo Schaper said "Mineralized water
with a low sediment load refracts light around inside the water molecules
(spread at a 105 degree angle HOH) which refracts back to human observer as
blue, with the intensity increasing with the length of the column to an almost
purple."
How does light refract around "inside" water molecules? I'd appreciate a
reference or two on this?
Jo also reported that distilled water in a long column is colorless. I'd also
dispute that. Get that column deeper than about a meter and a half, and it's
blue!
Then consider ice! Icebergs and glacier ice are REALLY blue! Trouble is,
there's usually blue sky and blue ocean or lake water to blame for presumed
reflections. But get to the snout of a thick glacier that isn't dumping into
a lake or ocean, and go out on the greyest day you can stand, and boy, that
ice is REALLY blue! Then chop that blue ice into small pieces that you can
hold in your hand, and guess what? The blue color just disappears, and you've
got a bunch of white to colorless ice in your hands!
--DWS
P.S. Liquid nitrogen is (in the quantities I've seen it) essentially
colorless, but if you are stupid enough or in risky employment enough to see a
liter or so of liquid oxygen, it's blue!
stev...@ibm.net
In your question on the blueness of water, you mention that you discount sky
reflections. I agree, however, I ask, why is the sky blue ? If you can
answer that then you can look to that answer for the blueness of water -
since the same effect takes place.
The same effect as in passing light through glass in fact.
Light has a different velocity in different mediums. If light passes through
glass you will see a dispersion, due to refraction, as it goes deeper into
the glass with red travelling faster (having a longer wavelength), i.e. the
principal of a prism. I was initially ready to say that is the reason for
seeing blue but now I think - where does the red go ? Since you can see all
the colours that make up white light as it is dispersed through a prism.
Why, if the light is dispersed, do we not see reds and yellows, etc, if we
stand in different positions ?
If you passed sunlight through blue glass, all you would see is blue light
at the far side, I think that the same principal might apply here also.
If you look at 'clear' glass in a window you will see light coming through,
looking white. If however, this glass was thicker, it would appear greenish
blue.
The same effect occurs, as you said, with water. And, I think, with air.
Could it be that the actual colour of water is a blue colour, albeit very
faint, and that as you thicken it (deepen would be a better word) the colour
becomes more noticeable ?
Could the presence of large amounts of water vapour in the atmosphere be the
reason for the blue colour of the sky ?
This I think brings you back to your first sentence, "because it's blue".
Sorry about that.
Steve Foster
Doug Shakel wrote in message <6rijpp$g0$1...@polaris.azstarnet.com>...
As the pool was filled, the blue color would become evident as
>the water was about 4 to 5 feet deep in the deep end
Christof Kuhn
passing glass or water.
The simple answer is, red light does not change its direction (so if you
look at the sun through 10m of water, it will still appear white (blue
plus green plus red)). In contrast, if you look to other directions
within the water, what would you see? If there was no reflection, you
would see black (i.e. nothing). However you see the blue part of the sun
light's spectrum. It results of all the sun beams that pass you
(respectively only the red light of the sun beams passes you). The blue
light is scattered all around.
This effect is caused by the features of the water molecule indeed.
--
Christof Kuhn, Univ. f. BoKu Wien, Austria
h944...@edv1.boku.ac.at
Homepage: http://homepage.boku.ac.at/h9440283
S Krueger
>Doug,
>In your question on the blueness of water, you mention that you discount sky
>reflections. I agree, however, I ask, why is the sky blue ? If you can
>answer that then you can look to that answer for the blueness of water -
>since the same effect takes place.
>Steve Foster
>
>Doug Shakel wrote in message <6rijpp$g0$1...@polaris.azstarnet.com>...
>As the pool was filled, the blue color would become evident as
>>the water was about 4 to 5 feet deep in the deep end
>>I don't buy sky reflections either. Go out on a really grey day and lower
>a white object in to clear clean ocean or lake water. It starts to look
>blue after it gets down 5 feet or so.
>>
>>As to "light scattering"... Well, in a sense that COULD be correct, but
>then that just begs the question of "why BLUE" instead of another color.
The "Why is the sky blue?" question relates to scattering of light by
minute paricles in the atmosphere. I believe it is called Rayleigh
scattering, but my memory of college physics is fading. The intensity of
scattering is a function of wavelength, with blue being scattered
preferentially. Thus, when you look at any part of the sky, you are
seeing more blue than red being scattered from the sunlight spectrum.
When the path of sunlight through the atmosphere is very long, as at
sunrise and sunset, the sun even appears red due to the preferential
stripping out of the blue end of the spectrum.
I recall an excellent lab demonstration where cigarette smoke, which
consists of minute particles, was shown to appear blue when illuminated
with white light in a dark room. But if the smoke is inhaled into the
lungs, where the minute particles accumulate water and get larger, and
then exhaled, the smoke was much more white. The increase in particle
size was sufficient to allow much greater scattering of the red end of
the spectrum. The thermal jostling of air molecules is only sufficient to
keep very tiny particles aloft for long, so the natural tendency is for
blue light to be preferentially scattered. The larger particles of human
smog are capable of scattering much more of the spectrum and cause a
white haze.
Curiously, the scattering of blue light by the atmosphere is done in a
partially polarized fashion. If you take a polarized filter, say
polarizing sunglasses, and look at a bright blue sky, you will see the
sky get lighter and darker as you rotate the filter. Photographers have
long used this effect to create deep blue skies in landscape photography.
The blue-green "color" of water or glass is caused by a similar effect.
Let me give an illustrative example. When you look into a rippling pond
with a sandy bottom on a sunny day, you will see the light and dark areas
on the bottom dancing about. But while the water all around appears to be
pale bluish green, the dancing points of light at the bottom appear
white. That implies that where the full intensity of sunlight is
transmitted, the color remains white. The pale blue-green everywhere else
is due to the preferential scattering of a small percentage of the blue
end of the spectrum. The color is more blue-green, rather than sky blue,
suggesting that the scattering in water is due to slightly larger
particle sizes, preferentially scattering light to a slightly longer
wavelength. It is not surprising that water is capable of suspending
slightly larger particles than air, so the wider spectrum of scattering
is predictable.
That the "color" of water and glass is caused, at least in part, by
preferential scattering of the blue-green end of the spectrum by minute
impurities is also demonstrated by the fact that ultrapure water shows
less of the effect, and fine crystal shows less of the effect than common
window glass.
Hope this clarifies some of the mystery. Now back to geology.
********************************************************
* S Krueger (skrueger at arco dot com) * This *
* This message is personal and does not * Space *
* reflect the opinions of my employer * For Rent *
********************************************************
josc...@aol.com
dsh...@azstarnet.com (Doug Shakel) wrote:
> I don't think I buy the various explanations that have so far showed up for
> this. I think the correct answer is on the order of "blue is the color of
> water". The trouble is, we are all usually very personally familiar with
> water every day, we are very aware of the hazards associated with water that
> isn't clear and colorless, and so if it's obviously blue, but know that's
> supposed to be ok, we'd like an explanation.
Unfortunately, because "blue is the color of water" is an example of circular
logic. "Why is the sun hot?" "Because the sun is hot," is a restatement, not
an answer.
<snip>
>As to "light scattering"... Well, in a sense that COULD be correct, but then
> that just begs the question of "why BLUE" instead of another color.
> Atmospheric scattering for blue skies (which itself takes a few knocks every
> couple of years or so) is usually ascribed to particulate content.
>
> But it's hard to blame particles in really clear, clean water.
It is the difference between sediment(particulate matter) and solution in
which the chemical molecules and ions are interspersed on an atomic level
with the water. Sediment hinders the blue effect, solution helps it.
> I'm also leary about the "Tyndall effect". Jo Schaper said "Mineralized
water
> with a low sediment load refracts light around inside the water molecules
> (spread at a 105 degree angle HOH) which refracts back to human observer as
> blue, with the intensity increasing with the length of the column to an
almost
> purple."
>
> How does light refract around "inside" water molecules? I'd appreciate a
> reference or two on this?
I'd suggest a good college physics textbook.
>
> Jo also reported that distilled water in a long column is colorless. I'd
also
> dispute that. Get that column deeper than about a meter and a half, and it's
> blue!
I've never seen distilled water in more than a watercooler sized jug. It was
clear, but there was also the glass container to contend with if you wanted to
apply a colorimeter.
> Then consider ice! Icebergs and glacier ice are REALLY blue! Trouble is,
> there's usually blue sky and blue ocean or lake water to blame for presumed
> reflections. But get to the snout of a thick glacier that isn't dumping into
> a lake or ocean, and go out on the greyest day you can stand, and boy, that
> ice is REALLY blue! Then chop that blue ice into small pieces that you can
> hold in your hand, and guess what? The blue color just disappears, and
you've
> got a bunch of white to colorless ice in your hands!
Well, nice observation, but it seems to make my point, not tear it down.
Thanks for your rebuttal, Doug. It's great to have some real discussion going
here. You know of course that color perception is in the eye of the beholder,
anyway.
Sincerely,
Jo Schaper
josc...@aol.com
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Chris Kauhi
S Krueger wrote in message ...
>In article <35dd3...@news2.ibm.net> , stev...@ibm.net writes:
>>Doug,
>>In your question on the blueness of water, you mention that you discount
sky
>>reflections. I agree, however, I ask, why is the sky blue ? If you can
>>answer that then you can look to that answer for the blueness of water -
>>since the same effect takes place.
>[snip]
>>Steve Foster
>
>That the "color" of water and glass is caused, at least in part, by
>preferential scattering of the blue-green end of the spectrum by minute
>impurities is also demonstrated by the fact that ultrapure water shows
>less of the effect, and fine crystal shows less of the effect than common
>window glass.
>
>Hope this clarifies some of the mystery. Now back to geology.
>
or...check out
http://www.cnmoc.navy.mil/educate/neptune/quest/seawater/blue.htm
Ejseger
it's the sun!) . .
We all know that water isn't *really* blue - for some reason it appears blue,
whether it's a sky reflection, the color of the pool walls, light refraction or
whatever. But I've had the experience of looking at a body of water and seeing
it as blue and then getting real close to it and finding it was clear or muddy.
So my brilliant idea is this: maybe the perception of color depends upon the
angle of viewing! If you stand at a distance from the water, it looks blue,
but if you look right down at it, it's clear (or muddy if it's the
Mississippi!).
Ellen
Jo Schaper
wrote:
The angle of view accounts for the sky reflection. It
doesn't account for what we have been discussing, since the
water in an underground pool (no sky) under artificial
light which is deep enough looks blue from any angle. The
exact color of blue does somewhat depend on the color of the
light (incandescent, flame or fluorescent) however.
regards,
Jo
Hoffman, Nick N
> Curiously, the scattering of blue light by the atmosphere is done in a
>
> partially polarized fashion. If you take a polarized filter, say
> polarizing sunglasses, and look at a bright blue sky, you will see the
>
> sky get lighter and darker as you rotate the filter. Photographers
> have
> long used this effect to create deep blue skies in landscape
> photography.
The polarisation is circular, centred on the sun. Bees sense this
polarisation and can find the sun even on a cloudy day - they use coded
dances to tell other bees where to fly to find rich crops of flowers.
The Vikings used calcite crystals (which are clear and have very strong
birefringence) to split the sunlight into its polarised components and
thus detect the location of the sun even in horrible North Atlantic
cloud. Thus their navigational prowess and the colony in Vinland.
Doug Shakel
It was my understanding the mineral the Vikings used was cryolite, rather than
calcite.
--DWS
William R. Graham
absorbs the other colors of the spectrum and reflects only the blue
wavelength. Hence we "see" blue water. Water is actually clear or
colorless.
Jo Schaper wrote in message <35e0e25b...@news.idt.net>...
Andy Christy
>
> In article <35E0FC80...@NOSPAM.bhp.com.au>, "Hoffman, Nick N" <Hoffman...@NOSPAM.bhp.com.au> wrote:
> >S Krueger wrote:
> >
[snip]
> >> sky get lighter and darker as you rotate the filter. Photographers
> >> have
> >> long used this effect to create deep blue skies in landscape
> >> photography.
> >
> >The polarisation is circular, centred on the sun.
Minor clarification: the polarization is linear, but the planes
of vibration for different propagation directions form concentric
circles around the sun.
>Bees sense this
> >polarisation and can find the sun even on a cloudy day
[snip]
Indeed. The little cleverbastids can not only see UV, but can also
detect the plane of polarization of light.
:-)
> >
> >The Vikings used calcite crystals (which are clear and have very strong
> >birefringence) to split the sunlight into its polarised components and
> >thus detect the location of the sun even in horrible North Atlantic
> >cloud. Thus their navigational prowess and the colony in Vinland.
> >
I don't think so . Beam splitting by calcite birefringence makes
for a cute effect if you look at some small marker (eg a dot on
paper) through it, but is not particularly useful for determining
the plane of polarization of light in the sky unless you have
an additional polarizer plate handy. Manufacture of the latter
would probably have been beyong Viking understanding of optics.
Try pleochroic properties - much more obvious (see below).
>
> It was my understanding the mineral the Vikings used was cryolite, rather than
> calcite.
>
Definitely not cryolite, which is almost isotropic, is colourless,
and is not found in large quantitioes in mainland Scandinavia
(they'd have to have got to Greenland before they discovered it!)
I heard that they used pebbles of gem-quality Norwegian cordierite,
which shows very strong cornflower-blue to pale yellow-grey
pleochroism that is easily visible to the naked eye.
I have tried this, and can vouch that it works.
Pleochroic red-green andalusite and various colours of tourmaline
also make good-ish sunstones if they are strongly coloured enough
for the pleochroism to be visible, but still transparent.
Strangely enough, standard ex-sunglasses polaroid does NOT actually
work as convincingly, probably because the hue does not change..
Andy C
BM223
>From: ejs...@aol.com (Ejseger)
>Date: 8/23/98 6:24 PM EST
>Message-id: <199808232324...@ladder01.news.aol.com>
>
>Something just occurred to me walking by the swimming pool in 105+ temps
>(maybe
>it's the sun!) . .
>We all know that water isn't *really* blue - for some reason it appears blue,
>whether it's a sky reflection, the color of the pool walls, light refraction
>or
>whatever. But I've had the experience of looking at a body of water and
>seeing
>it as blue and then getting real close to it and finding it was clear or
>muddy.
>So my brilliant idea is this: maybe the perception of color depends upon the
>angle of viewing! If you stand at a distance from the water, it looks blue,
>but if you look right down at it, it's clear (or muddy if it's the
>Mississippi!).
>Ellen
>
>
>
Seeing through the water like that is called Jacob's Window. If you can see
the fish, they'll be able to see you.
BM223