why does the sky and the ocean appear blue ?
Bigdakine
>Subject: why does the sky and the ocean appear blue ?
>From: sch...@spectra.net (Timothy Schmits)
>Date: Sat, 27 Sep 1997 22:47:41 GMT
>Message-id: <60k2h3$6...@ns2.spectra.net>
>
>thanks. I believe the answer has something to do with the atmosphere.
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Read up on Rayleigh scattering.
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Dr. Stuart A. Weinstein
Westchester Institute of Tectonics
"To err is human, but to realy foul things up
requires a creationist"
Uncle Al
Timothy Schmits wrote:
>
> thanks. I believe the answer has something to do with the atmosphere.
The ocean (a depth of clean water) is blue because vibronic coupling in
the water molecule extends its electronic absorption spectrum thoruhg
the infrared out to a 2.25 eV cutoff. Between 1.7 and 2.25 eV the molar
absorption coefficient drops from .01 to 001. It isn't much - hence the
need for depth of water or ice - but it is enough. By subtracting some
red light the net throughput appears blue. Tetraaquocopper(II) ion
tails from the IR to the visible in the same manner. Add ammonia and
the stronger lignd field intensifies the absorption and blue-shifts it.
The color of tetraminecopper(II) ion is quite saturated and lovely.
The sky is blue because of light scattering, both molecular and
particulate.
Hie thy buns to a libary (repository of thin glosses of information on
shredded, compacted, foliated dead trees) for "The Physics and Chemistry
of Color," Kurt Nassau.
--
Uncle Al Schwartz
Uncl...@ix.netcom.com ("zero" before @)
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Timothy Schmits
Todd Spohnholtz
On Sat, 27 Sep 1997 20:44:36 -0700, Uncle Al <Uncl...@ix.netcom.com>
wrote:
>Timothy Schmits wrote:
>>
>> thanks. I believe the answer has something to do with the atmosphere.
>
>The ocean (a depth of clean water) is blue because vibronic coupling in
>the water molecule extends its electronic absorption spectrum thoruhg
>the infrared out to a 2.25 eV cutoff. Between 1.7 and 2.25 eV the molar
>absorption coefficient drops from .01 to 001. It isn't much - hence the
>need for depth of water or ice - but it is enough. By subtracting some
>red light the net throughput appears blue. Tetraaquocopper(II) ion
>tails from the IR to the visible in the same manner. Add ammonia and
>the stronger lignd field intensifies the absorption and blue-shifts it.
>The color of tetraminecopper(II) ion is quite saturated and lovely.
M. Minnaert in "The Nature of Light and Color in the Open Air"
attributes the color of the ocean (and other large bodies of water) to
various factors. The first, and primary, to reflection of the sky
above. Next, Rayleigh scattering due to suspended particles in the
water. And also tertiary effects including: scattering (non-Rayleigh),
absorption, and to some extent fluorescence.
His work is quite dated though ... 1954.
___________________________________________________________
Todd Spohnholtz qroo...@wwa.com
Mechanical Engineering [remove 'q' to respond]
University of Illinois at Chicago
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cpo...@siue.edu
Siddharta Govindaraj
In article <60k2h3$6...@ns2.spectra.net>,
sch...@spectra.net (Timothy Schmits) wrote:
>
> thanks. I believe the answer has something to do with the atmosphere.
I believe the sky is blue due to Rayleigh scattering. I was told that the
ocean is blue due to scattering and also due to the Raman effect.Sir
Chandrasekhara Raman was awarded the 1930 Nobel Prize for Physics for his
work about this effect. You could find more about the Raman effect as I
dont know too much about it.
Siddharta Govindaraj
Siddh...@hotmail.com
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Phil Gibbs
Here is an article which will be in the FAQ soon
Why is the sky blue?
A clear cloudless day-time sky is blue because the oxygen and nitrogen
molecules in the air scatter blue light from the sun more than they
scatter red light. When we look towards the sun at sunset we see red and
orange colours because the blue light has been scattered out and away
from the line of sight.
<Picture>
The white light from the sun is a mixture of all colours of the rainbow.
This was demonstrated by Isaac Newton who used a prism to separate the
different colours and so form a spectrum. The colours of light are
distinguished by their different wavelengths. The visible part of the
spectrum ranges from red light with a wavelength of about 720 nm to
violet with a wavelength of about 380 nm with orange, yellow, green,
blue and indigo between. The three different types of colour receptors
in the retina of the human eye respond most strongly to red, green and
blue wavelengths giving us our colour vision.
Tyndall Effect
The first steps towards correctly explaining the colour of the sky were
taken by John Tyndall in 1859. He discovered that when light passes
through a clear fluid holding small particles in suspension, the shorter
blue wavelengths are scattered more strongly than the red. This can be
demonstrated by shining a beam of white light through a tank of water
with a little milk or soap mixed in. From the side the beam can be seen
by the blue light it scatters, but the light seen directly from the end
is reddened after it has passed through the tank. The scattered light
can also be shown to be polarised using a filter of polarised light,
just as the sky appears a deeper blue through polarised sun glasses.
This is most correctly called the Tyndall effect but it is more commonly
known to physicists as Rayleigh scattering after Lord Rayleigh who
studied it in more detail a few years later. He showed that the amount
of light scattered is inversely proportional to the fourth power of
wavelength for sufficiently small particles. It follows that blue light
is scattered more than red light by a factor of (700/400)4 ~= 10.
Dust or Molecules?
Tyndall and Rayleigh thought that the blue colour of the sky must be due
to small particles of dust and droplets of water vapour in the
atmosphere. Even today people sometimes incorrectly say that this is the
case. Later scientists realised that if it were true there would be more
variation of sky colour with humidity or haze conditions than was
observed so they supposed correctly that the molecules of oxygen and
nitrogen in the air are sufficient to account for the scattering. The
case was finally settled by Einstein in 1911 who calculated the detailed
formula for the scattering of light from molecules which was found to be
in agreement with experiment. He was even able to use the calculation as
a further verification of Avogadro's number when compared with
observation. The molecules are able to scatter light because the
electromagnetic field of the light wave induces an electric dipole
moment.
Why not violet?
If shorter wavelengths are scattered most strongly there is a puzzle as
to why the sky does not appear violet, the colour with the shortest
visible wavelength. The spectrum of light emission from the sun is not
constant at all wavelengths and is absorbed by the high atmosphere so
there is less violet in the light. Our eyes are also less sensitive to
those colours. That is part of the answer yet a rainbow shows that there
remains a significant amount of visible light coloured indigo and violet
beyond the blue. The rest of the answer to this puzzle is in the way our
vision works. We have three types of colour receptors, or cones, in our
retina. They are called red, blue and green because they respond most
strongly to light at those wavelengths. As they are stimulated in
different proportions our visual system constructs the colours we see.
<Picture>
response curves for the three types of cone in the human eye
When we look up at the sky the red cones respond to the small amount of
scattered red light and also less strongly to orange and yellow
wavelengths. The green cones respond to yellow and the more strongly
scattered green and green-blue wavelengths. The blue cones are
stimulated by colours near blue wavelengths which are very strongly
scattered. If there were no indigo and violet in the spectrum the sky
would appear blue with a slight green tinge. However, the most strongly
scattered indigo and violet wavelengths stimulate the red cones slightly
as well as the blue, which is why these colours appear blue with an
added red tinge. The net effect is that the red and green cones are
stimulated about equally by the light from the sky while the blue is
stimulated more strongly. This combination accounts for the pale sky
blue colour. It may not be a coincidence that our vision is adjusted to
see the sky as a pure hue. We have evolved to fit in with our
environment and the ability to separate natural colours most clearly is
probably a survival advantage.
<Picture>
A multi-coloured sunset over the Firth of Forth in Scotland.
Sunsets
When the air is clear the sunset will appear yellow because the light
from the sun has passed a long distance through air and some of the blue
light has been scattered away. If the air is polluted with small
particles, natural or otherwise, the sunset will be more red. Sunsets
over the sea may also be orange due to salt particles in the air which
are effective Tyndall scatterers. The sky around the sun is seen
reddened as well as the light coming directly from the sun. This is
because all light is scattered relatively well through small angles but
blue light is then more likely to be scattered twice or more over the
greater distances leaving the yellow, red and orange colours.
<Picture>
A blue haze over the mountains of Les Vosges in France.
Blue Haze and Blue Moon
Clouds and dust haze appear white because they consist of particles
larger than the wavelengths of light and scatter all wavelengths equally
(Mie scattering). Sometimes there may be other particles in the air
which are much smaller. Some mountainous regions are famous for their
blue haze. Aerosols of terpenes from the vegetation react with ozone in
the atmosphere to form small particles about 200 nm across which scatter
the blue light. A forest fire or volcanic eruption may occasionally fill
the atmosphere with fine particles of 500-800 nm across which are the
right size to scatter red light. This gives the opposite to the usual
Tyndall effect and may cause the moon to have a blue tinge since the red
light is scattered out. This is very rare, literally once in a blue
moon.
Opalescence
The Tyndall effect is responsible for some other blue coloration's in
nature such as blue eyes, the opalescence of some gem stones and the
colour in the blue jay's wing. The colours can vary according to the
size of the scattering particles. When a fluid is near its critical
temperature and pressure tiny density fluctuations are responsible for a
blue coloration known as critical opalescence (however, air density
fluctuations do not contribute the blue colour of the sky as sometimes
wrongly stated). People have also copied these natural effects by making
ornamental glasses impregnated with particles to give them a blue sheen.
Not all blue colouring in nature is caused by scattering. Light under
the sea is blue because water absorbs longer wavelength of light through
distances over about 20 metres. When viewed from the beach the sea is
also blue because it reflects the sky, of course. Some birds and
butterflies get their blue coloration's by diffraction effects.
Why is the Mars sky red?
Images sent back from the Viking Mars landers in 1977 and from
Pathfinder in 1997 showed a red sky seen from the Martian surface. This
was due to red iron rich dusts thrown up in dust storms which occur from
time to time on Mars. The colour of the Mars sky will change according
to weather conditions. It should be blue when there have been no recent
storms but it will be darker than the earth daytime sky because of Mars'
thinner atmosphere.
Phil Gibbs
http://www.weburbia.com/ http://www.weburbia.demon.co.uk/
"The difference between a flower and a weed is a judgement."
Phil Gibbs
Here is an article which will be in the Physics FAQ soon
Forgetu2
An experiment to do these effects can be done as follows:
Take a fish tank or long tank and fill it with water. Add about a half
cup of milk or so and stir it in. Get one of those overhead projectors or
slide projectors and shine it down the length.
The end...as viewed into the light will appear red, as a sunset. The side
will appear blue, as the sky.
-milk is a suspension of finely divided particles that scatter the light.
If memory serves correctly, the Rayleigh scattering is proportional to
1/frequency^4 - so blue light is scattered more than red light.
Jon
Michael J. Ramsey
>In article <60k2h3$6...@ns2.spectra.net>,
> sch...@spectra.net (Timothy Schmits) wrote:
>>
>> thanks. I believe the answer has something to do with the atmosphere.
>
>I believe the sky is blue due to Rayleigh scattering.
[snip]
Woods Hole Oceanographic Institute offers the following:
(http://www.whoi.edu/info/ocean-blue.html)
"Why is the ocean blue?
The ocean often looks blue because sunlight shines on tiny particles
suspended in the water. Along the shores of some areas, however, the
water looks green because the blue water is mixed with yellow pigments
present in floating plants.
Some oceans exhibit other colors:
The Black Sea looks black because it has little oxygen and a high
concentration of hydrogen sulfide.
The Red Sea looks red because it contains seasonal blooms of algae
that color the surface water red.
The Yellow Sea looks yellow because it contains a yellow mud carried
into it by adjoining rivers."
--Best regards,
--Mike Ramsey
Phil Gibbs
In article <34419d9a...@news.bellatlantic.net>, "Michael J.
Ramsey" <mjra...@bellatlantic.net> quotes
>The ocean often looks blue because sunlight shines on tiny particles
>suspended in the water. Along the shores of some areas, however, the
>water looks green because the blue water is mixed with yellow pigments
>present in floating plants.
Partly true, but even pure water with no suspended particles looks
blue. See the blue holes in Florida for example, where clear
spring water emerges from the ground. It is nor due to just
reflected light from the sky either. Pure water (and salt water)
absorbs red light and so appears blue.
Particle
>Partly true, but even pure water with no suspended particles looks
>blue. See the blue holes in Florida for example, where clear
>spring water emerges from the ground. It is nor due to just
>reflected light from the sky either. Pure water (and salt water)
>absorbs red light and so appears blue.
That kinda defeats that: "Pure water is colorless & odorless" thing...
Particle
bsp...@geocities.com
http://www.geocities.com/SiliconValley/Way/7650