Which colours of sunlight does sea water absorb first?

Pour sea water into a glass and it looks clear. Look out at the open sea and it is blue. Both are true, and the reason is sunlight.

Sunlight is made of all the colours mixed together. When it enters the ocean, the water soaks up, or absorbs, the colours at the red end of the light first. In this way the water acts like a filter. What is left, and what bounces back to our eyes, is mostly light from the blue end. A glass of water is too shallow for this to show, but a sea many metres deep has enough water to filter the light strongly.

The sea is not always blue, though. It can look green, brown or even red when light bounces off tiny particles, mud or living things floating in the water.

And most of the ocean is not blue at all, but completely dark. Hardly any sunlight reaches deeper than about 200 metres, and none at all reaches beyond about 1,000 metres. Below that lies a world of permanent night.