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Re: The blueness of water



On Fri, 26 Apr 2002, Larry Woolf wrote:

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?"

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.

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.

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.

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.

I mean the one's I've seen close up. The public pool back home was
painted sky blue, as was our own in-ground pool. Yes, it's hard to tell
the color from above if the pool has uniform depth. I assume that the
ones that I've seen from the air which have distinct color gradations
are painted white.


***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.

Great example! I might be wrong yet.

Now if we just had a side-by-side comparison of an ocean-water pool with a
fresh water version.


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