eli5: Why is water clear in small quantities but blue when in large quantities?
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Things appear colored to us because they absorb certain wavelengths of light. The light they don’t absorb is reflected back at us, and that’s the color we see.
Take a pane of colored glass that is slightly tinted blue. It looks clear, just a little blue-shifted. Now put another of the exact same panes of glass behind it. It’s a darker blue now. Then add another, and another. With each one the blue you see is going to deepen, because each pane of glass adds to the amount of non-blue light being absorbed.
The same thing is happening with water, however it reflects so little blue light that it appears clear in smaller quantities. Water itself is almost clear in general, the color we see in it is mostly from tiny impurities within it. So it’s not until there’s a WHOLE LOT of it that there’s enough blue light being reflected for us to notice it.
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