Beginner 8 minutes
Light underwater: why red disappears and everything looks closer
Absorption, refraction, and scattering, or why the sea looks blue and your hand looks bigger
At twenty meters, the blood from a cut looks dark green. The bright red fish in the photo you took with flash was, to the naked eye, a dull gray fish. It's not your eyesight: it's that the red simply wasn't there anymore.
In this article you'll understand the three phenomena that govern light underwater (absorption, refraction, and scattering) why colors are lost with depth, and why everything looks bigger and closer.
It explains how you really see underwater: why you need a torch in broad daylight, why distances deceive you, and why underwater photography demands what it demands.
Absorption: colors are lost in order
White sunlight is a mixture of all visible wavelengths. Water doesn't absorb them equally: it holds on to some longer than others, and it does so in a fairly predictable order.
Red is the first to disappear, within the first few meters. Orange and yellow follow. Green holds on considerably longer, and blue is the one that penetrates deepest. That's why water looks blue: not because it is blue, but because it's the color that survives.
This loss doesn't depend only on depth: it depends on the total distance light travels through the water. An object 5 meters deep but 10 meters away horizontally has already lost most of its red, because the light had to travel through 15 meters of water to reach it and return to your eye.
That detail explains something important for photography: getting closer to a subject recovers color far more effectively than increasing the light's power.
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