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 almost got it actually, but it isn’t reflection of the blue sky, but rather the ocean is blue the same way the sky is blue.

You have basically white light coming in from the sun (it has all the colors contained in it), but the way the atmosphere absorbs, transmits, and scatters the light means it ends up looking blue. (and at night, when the sunlight is traveling very far through the atmosphere, the blue light has scattered away and you are left with red light. That is why sunsets and sunrises are red, but at noon the sky above you is blue.)

Same thing in the ocean. Sunlight reflects off of it, but it also goes into the water and scatters and comes back up (this is why you can see into the water, and see rocks on the bottom. That is sunlight going down into the water, reflecting of the rock, and into your eye.)

The ocean looks blue because it scatters the blue part of sunlight, just like the sky does.