“Why is the sky blue?”
It’s one of those questions that most people don’t know the answer to, while others know at least the basic explanation that the atmosphere scatters blue light more strongly than the other visible wavelengths. Ask why that happens and you’ll enter the realm of some interesting and somewhat complicated physics. We’ll do that later, and do an easier one today.
Kids sometimes also ask why water is blue. The most common answer is that it isn’t – the blue of the ocean is attributed to reflected sky or something similar, while the water itself is clear. There’s some truth to that, but it’s not the whole story. Water really is blue – a very very faint almost transparent blue.
Let’s take a look at Wikipedia’s plot of the absorption spectrum of water:
It’s a logarithmic plot, so for instance water absorbs more than a billion times more ultraviolet than it does at the bottom of its transmission window. It’s no coincidence that visible light happens to be transmitted the best. There would be very little point in having eyes that can see wavelengths that don’t make from the sun through the atmospheric water vapor in any reasonable quantity.
But you may notice that although visible light is transmitted very well, blue wavelengths are transmitted better than red ones. After passing through a few dozen meters the difference is enough to be noticeable; much more of the blue light has passed through without being absorbed.
The qualitative reason for the gap is straightforward. The bending and stretching of the bonds between the hydrogen and oxygen typically have energies in the infrared and microwave frequencies, while the electronic energy levels typically have energies in the ultraviolet and higher. In the middle water just doesn’t have much internal structure with resonances near those frequencies and so most energy with those frequencies passes straight through.
And that’s why water is blue.