Re: The blueness of water

[Larry Woolf <larry.woolf@GAT.COM>]

-----Original Message-----
From:  William Beaty
Sent: Friday, April 26, 2002 2:54 PM
To: PHYS-L@lists.nau.edu
Subject: Re: The blueness of water


On Fri, 26 Apr 2002, Larry Woolf wrote:

> A sufficiently high concentration of colored particles or plant matter
> suspended in water will certainly give water the color of the particles or
> plants.
>
> But typical bodies of water are blue because water is a selective
absorber -

Last time this debate appeared on phys-L, I said what I'll say now:  the
swimming-pool reactor at Cornell looked very odd, it did not look the
color of swimming-pool water.  The walls of the tank were not painted
blue, and there were underwater floodlights.  The water looked like a
block of colorless glass.  It was only blue near the bottom (I vaugely
remember that it was 40ft deep, but I could be wrong.)

*** Here's another possibility.  If the reactor swimming pool was filled
with heavy water, it would be colorless
See:
http://webexhibits.org/causesofcolor/5B.html
"The graph at right gives the visible and near IR spectrum of H2O at room
temperature. The absorption below 700 nm in wavelength contributes to the
color of water. This absorption consists of the short wavelength tail of a
band centered at 760 nm and two weaker bands at 660 and 605 nm. The
vibrational origin of this visible absorption of H2O is demonstrated by the
spectrum of heavy water, D2O. Heavy water is chemically the same as regular
(light) water, but with the two hydrogen atoms (as in H2O) replaced with
deuterium atoms (deuterium is an isotope of hydrogen with one extra
neutron -- the extra neutron that makes heavy water "heavy," about 10%
heavier). Heavy water is colorless because all of its corresponding
vibrational transitions are shifted to lower energy by the increase in
isotope mass. For example the H2O band at 760 nm is shifted to approximately
1000 nm in D2O."


The occasional white-bottom pool is interesting because the shallow end is
far less blue than the deep end.

*** This observation is strong evidence for the color of water being due to
selective and gradual absorption: the longer path length of light passing
through the deep water and reflecting off of the white bottom leads to more
absorption of the red, hence the deeper blue color at the deep end.