Why is water in the ocean blue?

09 December 2007

Question

Why is water in the ocean blue when you look at it, but water in lakes and rivers looks clear?

Answer

This is all to do with how water interacts with light. When water is a vapour in the atmosphere, the molecules are not glued together in the same way they are in a liquid. Water molecules tend to stick together with hydrogen bonds, but in a vapour they can spread out. In this state they are very good at soaking up infra-red light, but not visible light.When water forms a liquid - lots of molecules stuck together - it stiffens the bond between the hydrogen and the oxygen, and instead of absorbing infra-red, it now absorbs light at the red end of the spectrum. White light is a mixture of lots of different wavelengths, and if you take out some of the red light, it looks a little bluer. The more water you have, the more red is absorbed and the bluer things look.

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