The next time you are standing
by a lake, pond or the ocean, ask
yourself why it looks the way it
does. What makes oceans and lakes
look blue or green and sometimes
black? Why do rivers most often
appear brown? When you cup some
of the water in your hand, it's
colorless, but look at the body of
water and it's a color. The most
obvious answer is that it reflects
the sky since you can see reflections
of clouds also. But reflection is only
part of the answer.
It is true that part of the light you
see is a surface reflection of sky
light. The amount of this sort of
reflection, though, depends on a
number of things such as your
viewing angle. On the other hand,
much of the color you see comes
from the water's depths.
White sunlight contains all colors..
Water absorbs some of these colors
more than others. But you need a
lot of water to notice this effect -
which is why water cupped in your
hand has no color at all. In a large
body of water though, the water
molecules quickly absorb red light,
but they don't absorb blue light
nearly
as well. So in this case, more blue
light is transmitted through the
water. It's actually the same
principle behind why the sky is
blue.
The blue effect is often increased
because ocean and lake water
usually contains floating particles
of dirt, and dead and living plants
and animals. They help reflect the
blue light back upward - so that the
lake or ocean looks blue. The blue-
green of tropical waters is due
entirely to absorption of red light
rather then suspension of particles.
It's a different blue than the blue
of the oceans off the northeast
coast, whose blue is caused by
suspended particles and plants.
So now we know why some bodies of
water appear blue, but what about
the other water colors, like gray,
brown and black? Different colors
can be produced by specific things
suspended in the water. Mud acts in
the opposite way from water -- it
reflects red light more than blue. So
muddy water looks brown. Since
rivers are big transporters of mud,
they usually appear brown. Peat
dissolved in water absorbs almost
all the light that touches it - so a
lake full of peat may look black. The
ocean often usually looks gray on a
stormy day. Partly, that's reflected
gray sky - but it's also because
clouds filter out a lot of the sun's
red light before it ever reaches the
water.
Colors of water can be pretty
distinctive. The Black Sea is so
named because of its color, a very
dark almost black color. In this
case, it's the almost constant fog
over the Black Sea that absorbs
incoming light and makes the sea
appear black. It was enough to make
the Turks and many other sailors
want to avoid what they saw as an
evil looking sea!
