text: The intrinsic color of liquid water may be demonstrated by looking at a white light source through a long pipe that is filled with purified water and closed at both ends with a transparent window. The light cyan  color is caused by weak absorption in the red part of the visible spectrum.
  Absorptions in the visible spectrum are usually attributed to excitations of electronic energy states in matter. Water is a simple three-atom molecule, HO, and all its electronic absorptions occur in the ultraviolet region of the electromagnetic spectrum and are therefore not responsible for the color of water in the visible region of the spectrum. The water molecule has three fundamental modes of vibration. Two stretching vibrations of the O–H bonds in the gaseous state of water occur at  = 3650 cm and  = 3755 cm. Absorption due to these vibrations occurs in the infrared region of the spectrum. The absorption in the visible spectrum is due mainly to the harmonic  = 14,318 cm, which is equivalent to a wavelength of 698 nm. In liquid state at 20°C these vibrations are red-shifted by hydrogen bonding, resulting in red absorption at 740 nm, other harmonics such as  giving red absorption at 660 nm. The absorption curve for heavy water (DO) is of a similar shape, but is shifted further towards the infrared end of the spectrum, because the vibrational transitions have a lower energy. For this reason, heavy water does not absorb red light and thus large bodies of DO would lack the characteristic cyan color of the more commonly found light water (HO).
  Absorption intensity decreases markedly with each successive overtone, resulting in very weak absorption for the third overtone. For this reason, the pipe needs to have a length of a meter or more and the water must be purified by microfiltration to remove any particles that could produce Mie scattering.