Hi, good question. There are sort of two answers.
One, water itself is not visible to humans or
fish, BUT we rarely find pure water in nature.
Usually, water has other things suspended in it,
like living things (bacteria, algae, etc.) or
particles (silt, mud, etc.). These thing influence
Water clarity is important because clear water
allows sunlight to go further down into water,
allowing for photosynthesis. It also
influences predator-prey interactions and
other behaviors. If water is very silty, it can
even clog gills.
While water is not visible, it does bend light.
This is called refraction. You can see this
when you reach into an aquarium or any body of
water. Your arm will not seem straight anymore.
refraction has some good images of how that
Can you think of other situations when
seeing underwater is important? What sorts
of things might cause your local water to be more
or less clear?
Thanks for asking,
Here are 74 answers to your question:
go to Quora site
I disagree with the answer that says our brains
just remove the seeing-air signals. Air
molecules are too small and too far apart for our
eyes to see them. Water molecules in water
are close together, but eyes can't see things
as tiny as water molecules, which would all
look the same and are all packed together.
Wikipedia has an article about fish vision, if
you want to learn more:
Vision in fishes
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