| Heavy water is colourless because all of its corresponding vibrational transitions are shifted to lower energy higher wavelength by the increase in isotope mass | any simple answer is bound to mislead |
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| International Organization for Standardization, ISO 2211:1973, Measurement of colour in Hazen units platinum-cobalt scale of Liquid Chemical Products• | the relative contribution of reflected skylight and the light scattered back from the depths is strongly dependent on observation angle |