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


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

I realize this isn’t very satisfying, but water looks blue because it is very slightly blue. Water absorbs other colors, but allows blue light to pass through, meaning it is blue. In small amounts of water the blue isn’t noticeable, but it becomes noticeable when there is more – that’s why the deep end of a pool looks like a darker shade of blue than the shallow end. You see the same thing with sheets of clear glass – a very thin sheet of glass will not seem to have color, but a thicker sheet will.

There are also generally impurities in water that will affect the color. In most swimming pools you have chlorine, which affects thing.

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.

This is video of a color chart being submerged up to 60′ in water. This shows how the other colors (red & green) are being absorbed by the thicker layers of water above the chart.


The more and bigger amounts of water that light has to pass through means more of the light’s color is changed by the water. Little water means little change to color. A lot of water means a lot of change to color.

It’s not the reflection of the sky. Indoor swimming pools with white tiles still look blue so it can’t be that right?

The reason is that water itself **is** blue, but only very faintly. So a cup of water still is blue, it’s just too pale for your eyes to notice it. When there’s a thicker path of water to look through like a big swimming pool, the pale blue is noticeable. And with an even thicker amount of water to look through like the ocean the blue looks even darker.

It’s like how looking through a pane of glass, glass looks clear. But if you look into the edge of a sheet of glass (so that you’re looking through several feet of glass) you can see that [it’s actually green](https://camblab.info/wp-content/uploads/2017/11/float-glass-300×193.jpg). It’s just so pale a green that light passing through only a 1/4 inch thick window doesn’t look very green.