eli5: Why is water clear in small quantities but blue when in large quantities?
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Water absorbs red and green more than blue, so when you have enough of it the reflected light is mostly blue.
Water is always *very very slightly* blue.
But its blueness is so small that you can only notice it if there’s a really really big amount of water.
Because water IS blue AND relatively transparent.
Water is blue because it absorbs colors other than blue much more. Thin layers of water doesn’t absorb much light so you don’t notice the color. As the layer gets thicker, more and more light gets absorbed and the color gets darker. If you get deep enough in the ocean, the water no longer looks blue–it looks black because almost all of the light (including blue) are absorbed.
Imagine a gray scale gradient from white to black. Replace the gray with the color blue. The bright side will still be white and the dark side will still be black, but in between you’ll see a lot a shades of blue.
Water is always blue, you only notice it when there’s large amounts of it because it’s only very slightly blue. Just like how it can be difficult to tell the color of a single piece of hair, but it’s easy to tell the hair color of many pieces of hair together.
The reason water is blue is because it absorbs the red and green colors of light better than the blue colors, which means blue is what gets transmitted or reflected back to our eyes.
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.
Saturation, basically. When there’s not a lot of water, we can mostly see entirely through it. The more water you stack up, the less we can see through and the more of the water’s color (or, much more commonly, the color of the things in the water) we see, until at some point enough of the light is blocked from coming back to our eyes that we can’t see anything but the color.
Water is the same color regardless of the quantity. It’s just more obvious the more of it you are looking through. It’s also easier to see if you’re looking at water in a white container. The reason is that the light coming in goes through the water, hits the white bottom, then travels back through the same water a second time. we have a white bathtub and you can see the blue water when it’s only half full.
Water is blue in all quantities, just very pale blue. So in small quantities it *looks* clear, but it’s still very pale blue. Just too pale to notice unless you have a much thicker path of water to look through like a swimming pool or lake.
This is very similar to how glass is actually green. It looks clear when you look through a thin sheet, but if you look [edge-on down a sheet](https://image.made-in-china.com/2f0j00BAOQenwPPHpd/1-8mm-3-2mm-4-8mm-Sheet-Glass-Float-Glass-for-Furniture-and-Building.jpg) or have [mirrors facing each other](https://www.reddit.com/r/funny/comments/zn3sju/looked_for_a_mirror_sub_but_apparently_theyre_all/), you can see that it’s actually green once the light has gone through enough glass for the faint tint to be noticeable to your eyes.
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