color of water
Unknown
In a moment of calm reflection, "John C. Randolph" <j...@idiom.com>
suggests:
>
> DB8088 wrote:
> >
> > My brother says the molecular make-up of H2O dictates its color to be blue. My
> > other brother says it's clear. Please, any comments?
> Pure water is basically colorless across the visible spectrum.
Actually, it's "transparent," meaning visible light tends not to be
reflected or absorbed by it. But (see below), note the word "tends."
In small quantities it appears "colorless" because it isn't reflecting
very much light.
> Large bodies of water sometimes appear blue, because they're reflecting
> the sky.
Not exactly -- Light is not *actually* colorless, in the sense that it
reflects *no* light (or all light equally well). It does absorb some
of the radiation that falls upon it, chiefly in the longer-wavelength
parts of the spectrum. It also reflects some of the light that falls
on it, but not much. Light pretty easily passes through small
quantities of it.
In the case of large bodies of water, some of the blueness may be due
to the reflection of the sky, but if you've ever been out at sea on a
cloudy day, you'll note that the water is quite blue even on those
days.
Probably the bulk of the blue that we perceive in water is due to the
above absorption/reflection characteristics. As light passes through
water, it progressively becomes constituted of shorter-wavelengths
because the longer-wavelengths are absorbed. Some few of these quanta
are reflected back in the direction from which they came, but most are
scattered back obliquely (which is why the water is more pale around
your shadow). These are then further filtered by the water, so that
by the time the light (that has been filtered going down and filtered
coming back up) gets to your retinae, it's almost exclusively from the
short-wavelength end of the spectrum.
Shorter wavelengths we perceive as blue.
In shallow or murky water the likelihood that it appears blue due to
the sky is, as I said, somewhat greater. But, murky or shallow water
on a cloudy day is more likely to appear as *green* than blue. The
demonstration of this is left as an exercise for the reader... ;)
Cheers,
m
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
Marc Carter, PhD Assistant Professor,
Dept. of Psychology Itinerant Scientist and
Hofstra University Inveterate Skeptic
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
"The data are always right, but they are not
always the right data."
R.S. Malpass
Rob and/or Kristin Spencer
However,
when the "thickness" is significant enough (like in a pool most likely), the
slower
frequencies of light (red, orange, yellow) are absorbed by water leaving
only the
faster frequencies (like blue which our eyes are most sensitive to compared
to violet)
to reflect off of the white surface at the bottom of the pool.
Tay wrote in message <38ad4...@news.cyberway.com.sg>...
>> Large bodies of water sometimes appear blue, because they're reflecting
>> the sky.
>
>Apart from this effect, I think water IS bluish (scatters or absorbs more
>red or something). I'm no expert, just suggesting that the above may not be
>the whole story.
>
>I've a swimming pool at home which is tiled with a "background" of white
>tiles with "lines" of light blue tiles.
>
>But when filled with water, the white tiles appear light blue, and the
light
>blue tiles appear a deeper blue. And from my experience, water always looks
>bluish, (when there's enough of it to look through).
>
>Perhaps this is due to impurities.. I dunno.. never had a large amount of
>"pure water to look though.
>HTH
>
>dt
>
>
John C. Randolph
Marc Carter wrote:
> In the case of large bodies of water, some of the blueness may be due
> to the reflection of the sky, but if you've ever been out at sea on a
> cloudy day, you'll note that the water is quite blue even on those
> days.
Actually, I've noted just the opposite. When the sky is cloudy over
Half Moon bay near San Francisco, the water looks grey, too.
-jcr
--
"The US Constitution may not be perfect, but it's better than what we
have now!"
Stephen
> DB8088 wrote:
> >
> > My brother says the molecular make-up of H2O dictates its color to be
blue. My
> > other brother says it's clear. Please, any comments?
>
> Pure water is basically colorless across the visible spectrum.
>
> Large bodies of water sometimes appear blue, because they're reflecting
> the sky.
Not actually true- water in bulk _does_ genuinely have a blue colour.
--
Felix qui potuit rerum cognoscere causas - Virgil.
Marc Carter
>
> Actually, I've noted just the opposite. When the sky is cloudy over
> Half Moon bay near San Francisco, the water looks grey, too.
>
Sigh. It's what I deserve for growing up on the Gulf Coast. Water's
*always* gray, there.
I confess I don't know why the clear cold Pacific would look gray,
unless it's reflecting the sky, so I'm going to go with you on that
one and say that that has something to do with it. But I'll stick to
my earlier guns and say that in the main, though, the blueness is from
the absorption and scattering properties of water, not from it
reflecting the sky.
Rob and/or Kristin Spencer wrote:
>
> Small "thicknesses" of water are transparent to all frequencies of light.
> However,
> when the "thickness" is significant enough (like in a pool most likely), the
> slower
> frequencies of light (red, orange, yellow) are absorbed by water leaving
> only the
> faster frequencies (like blue which our eyes are most sensitive to compared
> to violet)
> to reflect off of the white surface at the bottom of the pool.
You probably mean "higher" and "lower" frequencies (shorter and longer
wavelengths). Unless Einstein did something very wrong, none are
faster or slower than the others, if they're in the same medium...
Our eyes are only a tiny bit more sensitive to blue than violet, but
they are much much less sensitive to these than they are to reds or
yellows or even greens. And the violets are even more likely to get
reflected back to your eyes than blues are -- they're even shorter
wavelengths. So sensitivity really hasn't much to do with the
blueness of water.
:)