Water eats red light
Why Is the Sea Blue?
Water absorbs red light within a few metres, so only blue survives deep enough to be scattered back.
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Step 1 of 5
A glass of water looks perfectly clear.
Over a few centimetres water absorbs almost nothing you can notice, which is why people assume it has no colour of its own.
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Step 2 of 5
Over metres, it absorbs red strongly.
Water molecules absorb long wavelengths far more readily than short ones, so red light disappears within the first few metres.
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Step 3 of 5
So only blue light travels deep.
By ten metres down there is essentially no red left. Everything is lit by the blue and green that made it through.
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Step 4 of 5
Some of that blue scatters back to you.
Particles and the water itself scatter a fraction of the surviving light back upward, and that returning blue is what you see.
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Step 5 of 5
Reflection and contents change the shade.
A calm sea does mirror the sky, and mud or algae shift it towards brown or green, which is why the colour varies so much.
Divers find that blood looks green below about ten metres, because there is no red light left for it to reflect.
The short version
The sea is blue because water genuinely absorbs red light within metres, leaving only blue to travel deep and scatter back to your eyes.
Try it yourself
Look at a long white bath or a deep swimming pool. The water takes on a blue tint over distance, with no sky to reflect.
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