eli5: Why is water clear in small amount but blue in large amount like an ocean?

Question

I thought it might be the reflection from the sky but if that was the case, why does the ocean appears more blue the deeper you go?

In: 2918

Answer 1

You know how when light passes through colored glass, the light that passes through is now that color? What’s going on is that all the *other* colors/frequencies of visible light are being absorbed by the glass, or being re-released as that color. For colored glass, this effect is pretty dramatic- stained glass has strong colors because the metals injected into the glass are *really good* at absorbing light of specific colors.

*All substances* that light passes through do this. There is no substance that is *perfectly clear*. But there are substances that aren’t very dense, or just absorb *very little* light. Our atmosphere is the former, and water is the later. Both take light traveling a pretty significant distance for our eyes to see their effects on light- and, as it turns out, both make that light tend towards the color blue.

Water absorbs Red light the most easily, then greens, and blue last. People see this effect the most strongly when scubadiving, because you can see the colors bleed out of the world as you go deeper and deeper, until barely any light at all is getting to that depth (not that you should be scuba diving to those depths, but you can definitely see the color red grow faint at reasonable diving depths).

So when you look at water in a clear glass, or water spraying out of a hose, those *look clear* because there just isn’t enough water for them to absorb a noticeable portion of the light passing through them, though they can definitely refract that light into rainbows and such.