Re: The blueness of water

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

-----Original Message-----
From:  William Beaty
Sent: Friday, April 26, 2002 2:54 PM
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 -

I don't know if this is true.  "Typical" bodies of water, i.e. the ocean,
are blue because of impurities.

***Not true ... or as John Denker would say, "What is your evidence for
this?"

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.

*** 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. There are also minimal reflections if the source of light
is in the pool, compared to sunlight incident on a pool, where the path
length of light in the water will be at least twice the depth of the pool
(most of the light will reflect off the bottom and much light will be
scattered so that the path length will exceed twice the pool depth).

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.)

*** Also incandescent lights are enriched in red light compared to sunlight
(2500K black body compared to 6000K black body).  If the water absorbed the
red preferentially, you would see the light as more white compared to sunlit
illumination for the same path length of light.

Also: nearly all the swimming pools I've seen have been painted sky blue.
*** I highly doubt it. It is difficult to tell the color of pools from an
aircraft.

This is quite apparent when looking at suburbs out of aircraft windows.
The occasional white-bottom pool is interesting because the shallow end is
far less blue than the deep end.

***Do a survey of pools in your area. Do the experiment and see.

***For an example, take a look at an empty and filled pool at:
http://www.sci-ed-ga.org/pdfs/apsclrpres042501.pdf
slides 44 and 45.