What we see when we look at water is rarely water. It may be a reflection of the sky, if we’re looking across it, or the bottom of a shallow pond if we peer down into it. It could be the bubbles on the surface after a wave breaks or the rainbow colours formed as a drop of oil spreads across a city puddle. But none of these is water, because pure water is transparent and acts like a window or a mirror. We notice more dirty windows than clean ones.
If you spot shadows on water, that is a sign that the water isn’t pure. Shadows appear on water only when light reflects back off particles suspended in it. The next time you walk over a low city bridge on a sunny day, look down into the water with a high sun on your back and you will see the shadow of the bridge as well as your own shadow in the murky water below you.
City rivers are full of particles, not always bad ones, often just mud and silt in suspension, but you won’t see pure water under a town bridge. The sunlight bounces back off these particles into your eyes giving a bright colour that contrasts with the darker, duller shade where there are shadows. If there were no particles at all in the water, if it was 100 per cent pure, the light would keep going until it hit the bottom and create a very different effect. It would appear odd, strangely pure, and everyone would comment on it. We have a strong sense of when we are looking at clear and impure water, which doubtless stems from our ancestral need to gauge the health of water sources.
Transparent water can be pure or dangerous, as some harmful things, including viruses, are too small to be visible to the naked eye; cloudy water is obviously impure and contains something we might not want in our stomachs.
An excerpt from the book, The Hidden Seasons, by Tristan Gooley.
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How to Use Your Hands to See Underwater
The Colour and Shape of Navigation Marks
How to Read Water – The Book