|I would like to know why water's blue in the
ocean and clear in a cup? My teacher lives in by
the beach and the color are green, red, gray,
black at night. My friend Valerie said that it
looks blue because of the sun reflection. What is
the waters real color?
|Question Date: 1998-01-23|
Actually, there are several answers to this
question-- but I'll start by saying that your
friend is basically right, the ocean is blue
primarily because you see a reflection of the sky
which is usually blue. (However, as you notice, at
sunset, the ocean reflects the sunset colors of
red, orange, ping, and yellow.) If you take your
cup of water outside, what color does the top
appear? (Remember that at the beach you can only
see the top of the ocean.)(If you have a pair of
swimming goggles, you can hold your breath and see
the color of the ocean from under the surface. --
It may surprise you!).
A second answer is
that a large volume of water is indeed very
slightly blue-- but you won't see this effect in
pure water unless you have a very large tank --
tens of feet long. Most of the color you see this
way is due to water impurities -- even purified
water has dissolved oxygen which can form very
light blue complexes.
Finally, even small
particles of dust (which aren't colored) in large
volumes of "clear" fluids will have a bluish cast
due to the physics of light scattering. (this is
why the earth's sky appears blue). However, there
is a lot of dust on Mars-- have you seen the
pictures of the sky there taken recently? Be sure
to check that the photo is "true color" but the
color of the dust and its size can make a difference.
Water absorbs more of red light than it does
blue.To actually see a significant difference with
our eyes the light must travel about thirty feet
in the water. Since the sunlight contains all
colors, if you look at water more than about
fifteen feet deep it looks blue. However a cup of
water looks clear to us because it is so very
slightly blue that our eyes cannot see
Now all this works if the light falling
on the water contains all colors. If it doesn't as
maybe in the evening or night, then the color of
the water changes accordingly to green, red gray,
black; just like a red cloth in a blue light looks
black! So color also depends on what light you are
So now, think about whether
water has a "real" color...
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