Re: The blueness of water

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

At 12:13 AM -0700 4/28/2002, William Beaty wrote:
> Heh.  What's your evidence that the mineral impurities in ocean water are
> colorless?  The water is full of dissolved salts.  Just to clarify, we're
> arguing whether their color effects as viewed through a few meters of
> ocean water is significantly smaller or larger than the color effects
> caused by the water itself.

Since water - fresh or salt - generally appears colorless for shallow
depths, blue-green for a couple of meters depth, and blue for more than a
few meters, I would argue that dissolved salts are not a major effect.  The
salts most likely lead to increased scattering of incident light, which
would affect the path length of light in water before it enters your eye.
Certainly if there is a high concentration of colored matter or
particulates, this can dominate the intrinsic color of water.
By the way, using the optical constants of water,  I developed an exercise
where a student can demonstrate how the color of water changes with
increasing depth (assuming that the light reflects off of a reflective
bottom).  It shows quite nicely how water changes from colorless to
blue-green to blue with increasing depth. You just use the n and k values
that I showed in a previous post in conjuction with the color picker that
is built into Word and most other programs.
Also, I live in the land of 10,000 pools. I have never seen a pool painted
blue.  They all have white sides.  But the color of the bottom is critical
for attaining the desired water color.  A dark bottom will lead to a pool
that is dark blue - almost black.  A white bottom will lead to a light blue
or blue green color.  A grey bottom will lead to a more classic blue pool
color.
Note that if you have a fountain and want the water to appear blue, you
would paint the fountain walls and bottom blue, because the depth of the
water in a fountain (or glass of water) is insufficient for the intrinsic
absorption of water to be noticeable.

One other key factor that others may not be aware.  The reflectivity of the
water surface is a very strong function of angle.  Water (or glass or
plastic) reflects about 4% of the light at normal incidence, but  that %
increases greatly with angle.  At steep angles, its reflectance is very
high - which is why you can see the reflectance of mountains and sky in
lakes and still bodies of water.   That is the origin of the common
misconception that water is blue because it reflects the color of the sky -
which is stated in some "ask a scientist" sites on the web.
----------------------------

Earlier, I wrote:
> > *** If there were underwater floodlights, then the path length of light
> > passing through water and then entering your eye is certainly less than the
> > depth of the pool.
Bill  then wrote:
> Ooo, good point.  I never noticed the halved path length caused by
> underwater lighting.  I think the light is still decreasing linearly at
> those depths, so an internally-illuminated pool 10 meters deep would look
> approximately as blue as a top-illuminated pool 5 meters deep.
Brian then wrote:
My attention monitor tripped on this linear suggestion. I assume
by contrast, an exponential decrease of light in an ordinary absorber.
Who will recall the relevant facts for me, please?

The absorption is not linear but exponential. Brian is correct.

Dr. Lawrence D. Woolf; General Atomics, 3550 General Atomics Court, Mail
Stop 15-242, San Diego, CA 92121