Water is blue … because water is blue

Actually, water is quite a transparent liquid, but not perfectly transparent. All substances to a certain degree absorb light, and as a consequence, the intensity of a beam of light spreading through matter drops exponentially with distance, as described by the so-called Beer-Lambert law. Pure water appears transparent because it takes a distance of the order of metres to reduce by half the intensity of light passing through it. And, what is most important for the apparent colour of water, the absorption depends on the wavelength of light, hence colour.

And, as it turns out, heavy water isn’t blue.