I love my colleagues at NPL.
First of all James Miall sent me a link to some measurements of the transmission spectrum of water H2O and heavy water D20. The measurements are relatively simple and the paper is well written and straightforward.
Then Jenny Wilkinson took me to one side and very patiently explained to me how to clearly see the blueness of water. She filled a white polystyrene box with water – and viewed against the white of the box the water was clearly blue. I have tried the experiment at home and took photographs of a polystyrene box empty, with 5 cm of water , and with 10 cm of water. I have combined the three pictures into a single composite picture (above). Even allowing for the vagaries of colour reproduction in cameras and screen monitors, the water appears to be blue! At least it does around the edges.
So now I am trying to understand why water looks bluer when viewed against the sides of the box, but appears essentially clear when viewed against the base of the box. I think that it is because the materials of the wall appears less bright but when I try to get quantitative, it doesn’t make sense.
I think I will just leave the picture here and hope that another of my colleagues can explain the observation.