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Why does the concrete look darker around the pool when it gets wet ?
I'm not really sure, but here's a good guess. Light usually reflects
from the concrete surface becasue there is an abrupt change in how
light transmits in air to how it transmits (over very short distances)
in concrete. That's the same reason why some light reflects off glass
or other surfaces. If the transition from one medium for the light to
another is made less abrupt, then the total amount of reflected light
goes down. Lenses, for example, are often coated with a thin layer of
material in which light travels slower than in air but faster than in
the lens, to reduce reflection. Water has an "index of refraction"
higher than air, but less than almost any rock, and concrete is made
largely of rocks. So my guess is that it acts as an anti-reflection
coating, keeping the light from bouncing back off the surface. If
somebody knows more definitely what the main effect is, please let us
(published on 10/22/2007)
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