Transparency of Water in the Visible Range

Water is strongly absorbing at most of the wavelengths in the electromagnetic spectrum, but it has a narrow window of transparency which includes the visible spectrum. The span of the absorption spectrum shown is from wavelengths on the order of a kilometer down to about the size of a proton, about 10^-15 meters. It doesn't absorb in the wavelength range of visible light, roughly 400-700 nm, because there is no physical mechanism which produces transitions in that region - it is too energetic for the vibrations of the water molecule and below the energies needed to cause electronic transitions.

The window of transparency of water matches the peak emitted intensity of a blackbody radiator at 6000 K, a reasonable model for the output radiation of the Sun. The absorption spectrum of water and the peak of the Sun's radiation are of course physically independent phenomena, but it is fortunate for life on the Earth that they coincide. Just above the visible in the ultraviolet, the absorption of water increases by nine orders of magnitude, adding to our protection against ultraviolet rays from the Sun.

The absorption of electromagnetic radiation by water spans a wide range of physical phenomena, characteristic of the general interaction of radiation with matter. It absorbs strongly in the microwave region by excitation of molecular rotations. In the infrared it exhibits strong absorptions from vibrations of the water molecule. As you go above the visible through the UV toward x-rays, it successively absorbs by photoelectric effect, Compton scattering and finally pair production.