Why the Ocean Is Blue

The board draws the scene: the sky-reflection story crossed out, a shaft of white sunlight plunging down and losing its red end rung by rung, a diver’s flare fading from red to grey on the way down, and the sky and the sea set side by side at the bottom. This lesson works out exactly how a beam of sunlight loses its colour, one metre at a time.

Before you start. You need nothing but a swimming pool and a torch; there is no maths in the story half of this lesson. Why the Sky Is Blue already explains why the sky is blue, and this is deliberately a different story - the sky’s blue comes from light bouncing around in air, the sea’s blue comes from water itself swallowing a colour.

The idea in 30 seconds

The ocean is blue because water absorbs red light far more than blue light, not because it reflects a blue sky.
Absorption is a material holding on to some colours of light and letting others through; wavelength is the spacing of a colour’s ripples, and water swallows red’s long wavelength far more easily than blue’s short one.
Absorption alone would leave the sea black; scattering, light bouncing back off water’s own molecules and favouring blue, is what actually reaches your eye.
A glass of water is one thin layer and looks clear; a pool is a few metres and looks blue; the ocean is thousands of metres and nothing but blue survives.
Raman proved the mirror story wrong with a polarising prism in 1921, and seven years later discovered the very effect, molecules shifting scattered light’s colour, that explains why.

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