A video of a diver’s finger spurting green blood, not red, has gone viral. The explanation, however, is not supernatural, but a refraction of light.
This sight may seem startling at first to the average person, but to the experienced diver, it is no surprise. Our blood is not color-changing, of course, and when it returns to the surface it will be back to its original color. So it is not a color change to green, it is merely a loss of gloss. But how is this possible, and how many meters down does this curiosity find its way into the water?
As we dive deeper and deeper into the sea, the colours seem to gradually change. Light travels much faster in the air than in water – everything slows down, not just our movement. And different coloured lights travel at different speeds because they have different wavelengths. Red is particularly sluggish in this respect, so it is almost invisible underwater. At a depth of about 10 to 15 metres, it has no chance of showing itself, because the wavelength of red light does not reach that far and there is simply nothing to reflect it. In contrast, the wavelengths of blue and green light can be a joy to watch at this depth, and if you paddle down, black will dominate.
So don’t be surprised if you happen to get injured underwater, the blood that spills out will be bright emerald green.