Tools · Color & depth

Where do the colors go?

Color is just the wavelength of light your eye happens to catch. The red end of the spectrum is the long, lazy waves — around 700 nanometres — and the blue-violet end is short and tight, around 400. Since all light travels at the same speed, a long wavelength means a low frequency (red) and a short wavelength a high frequency (blue): wavelength and frequency are just two ways of describing the same wave, tied together by:

And frequency sets one more thing: energy. Each particle of light carries energy equal to Planck's constant times its frequency:

So the high-frequency blue end delivers more energy per photon than the slow red end. Wavelength, frequency, energy: three names for the same dial.

Water does not treat those waves equally. It drinks up the long, low-frequency red end almost greedily and lets the short, high-frequency blue end slip through — so as you go down, the spectrum gets eaten from the red side inward. Drag the depth slider and watch.

Here's the why, and it's not your eyes giving out. A water molecule is two hydrogen atoms bonded to an oxygen, and those bonds vibrate at frequencies that happen to line up with red and infrared light — so they soak that light up and turn it into a little heat. Blue light is too high-frequency to be caught that way, so it passes through. Water's transparency is best around blue (~450 nm) and falls off a cliff toward the red. Stack up enough metres of it and the red is simply gone.

So it's tempting to say higher energy, deeper, and among the colors you can see it holds — the high-frequency blue carries the most energy per photon and reaches the furthest down. But the energy isn't muscling its way through. Water simply happens to be clearest right around blue; push past it into the even-higher-energy ultraviolet and the water grabs the light again, so the deepest-reaching light is blue, not the most energetic. The giveaway that it's no law: in air the rule flips. There it's the energetic blue that gets scattered away — which is exactly why the sky is blue and the sunset red — while the low-energy red sails straight through. Same light, opposite outcome, because the mechanism switched from absorption to scattering.

That's why a red object looks brown, then gray, then near-black as you descend — there's no red light left for it to bounce back. Cut yourself at 30 m and the blood looks dark green, not red, until someone shines a light on it. And that's the trick: a dive light or a camera strobe carries its own full spectrum, and from a few feet away that light hasn't lost its red yet — so the colors snap back. Flip the dive light on above and watch. Colour is not the only sense the water rewrites — it also bends light at your mask and scrambles where a sound comes from.