To our knowledge the intrinsic blueness of water is the only example from nature in which color
originates from vibrational transitions. Scientists know the cause of color is atomic vibrations because "heavy" water (which vibrates more slowely) has a similar absorption curve, shifted to longer, slower frequencies. Water owes its intrinsic blueness to selective absorption in the red part of its visible spectrum. The absorbed photons promote transitions to high overtone and combination states of the nuclear motions of the molecule, i.e. to highly excited vibrations.3