What is the colour of heavy water? As in, what is the spectrum of backscattering fraction? What is the wavelength with highest backscattering fraction, and what is that fraction? As is well known, light water is neither colourless nor white, but blue and quite dark. The absorption spectrum of light water can be found here: http://en.wikipedia.org/wiki/File:Water_absorption_coefficient_large.gif Why is light water blue? Because the O-H vibrations efficiently absorb electromagnetic waves. The main absorption frequency is about 2900 nm, in infrared. But it has overtones. Also in infrared, 1450 nm (first overtone) and 970 nm (second overtone). And in visible light, conspicuous series of higher overtones. Absorption line at 740 nm (third overtone). Line at 605 nm (fourth overtone). Line at 515 nm (fifth overtone). Line at 450 nm (sixth overtone). There are also weaker lines clearly visible at 660 nm and 550 nm. Presumably overtones of some other vibration modes. Oddly, no line detectable between the 450 and 515 nm ones. And another weak line at 425 nm. And then a large increase of absorption blueward of 415 nm. With an obvious line at 395 nm. What is the absorption mechanism creating the 395 nm line? Now, D has much bigger reduced mass. Thus, heavy water has much longer infrared lines - about 1,4 times longer. We should expect heavy water to have sixth overtone absorption line at 630 nm, corresponding to the 450 nm line of light water. And that leaves the puzzle. Exactly how much light does heavy water absorb between 630 nm (analogic to the 450 nm line of light water) and 400 nm (the ultraviolet and violet absorption is presumably similar?)? The 425 nm line would have analogue at 595 nm. Next? Presumably higher overtones. But while the seventh overtone should be somewhere around 550 nm, every previous overtone was weaker than preceding. Eighth overtone is around 490 nm, etc. How low does the absorptivity of heavy water get at a minimum? And what is the wavelength where that minimum is?