Why is the sea blue? is a question one could ask when looking at a glass of clear water.

Anything Blue review – an once of physics, a bit of biology: why is the sea blue?  

A BBC article
published in the Earth section comes back on the whys and hows, with two explanations about the blueness of the sea: the blue light (1) and the green phytoplankton (2).

1. Natural light is composed of short and long wavelengths, each wavelength defining a colour – the same colours as in a rainbow. These wavelengths range from the ultraviolet (short wavelengths) to the infrared (long ones).

Light = violet + blue + green + yellow + orange + red

Because water absorbs the longest wavelengths (red, orange, yellow, green), only the short ones (blue, purple) are reflected in water.

Light + Water = violet + blue + green + yellow + orange + red

2. Not only water doesn’t absorb blue light, but some water is also ‘contaminated’ by phytoplankton, organisms who don’t absorb green light – instead, they feast on red and blue light to produce half of the oxygen we breath. As a result, the sea looks greener where phytoplankton thrives. 

Light + Water + Phytoplankton = violet + blue + green + yellow + orange + red

So, everything crystal clear? Joking. Want to know more about this fascinating subject? Read below.


A word from the BBC: ‘Pure water is of course clear. However if it is deep enough, so that light cannot reflect off the sea floor, it appears dark blue. This is largely because of some basic physics.

Human eyes contain cells capable of detecting electromagnetic radiation with wavelengths between around 380-700 nanometres. Within this band, different wavelengths correspond to the different colours we see in a rainbow.

Water molecules are better at absorbing light that arrives in longer wavelengths, meaning the reds, oranges, yellows and greens. This mostly leaves the blues, which have shorter wavelengths. As blue light is less likely to be absorbed, it can penetrate to deeper depths, making deep water look bluer.

Light at a short wavelength is also more likely to be scattered or deflected in different directions, including back out of the water towards our eyes, making the sea appear blue.

However, the purity of sea water varies. Particles suspended within it can increase the scattering of light. Sand and silt carried into the sea from rivers, or kicked up from the seafloor by waves and storms, can affect the colours of coastal waters. And organic detritus such as decayed plant matter – known to scientists as colour dissolved organic matter – can also complicate the picture, by adding greens, yellows or browns.

That’s the physics. But even more important is the biology, because the biggest impact on sea colour is made by tiny organisms called phytoplankton.’

Anything Blue review - an once of physics, a bit of biology: why is the sea blue?

A phytoplankton bloom in the Baltic sea, credit USGC @BBC – Anything Blue review – why is the sea blue?

Source: Is the sea really blue?@ BBC – Earth (featured picture credits: NASA)

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