Have you ever heard of light absorption? And if, do you still remember what does it mean? We all know the ocean is mostly blue but there might be a chance that we do not really know how to explain why. There must be something with the sky’s colour reflection and the solar light, for sure!

Some days when it rains, the ocean seems grey and all sad, other days, on the other hand, when the Sun is out, it has this intense blue colour that unconditionally attracts the human eye. So the sky’s colour reflection must really be the case. As much as it is true, it also isn’t completely right.

The solar radiation is another key component for the determination of the ocean’s colour. It comprises the ultraviolet (UV), visible (VIS) and the infrared (IR). The visible radiation has a wavelength (λ) between 400 and 700 nm and it will be presented here as a spectra of three main colours – blue, green and red. However, you can already deduct that the different wavelengths of the visible appear as a rainbow.

When the sunlight touches the oceans’ surface, some of the light is reflected, meaning that it scatters back into the atmosphere, while the majority of it is absorbed directly by water molecules, organic compounds, their chemical bonds and phytoplankton. Some of the blue wavelengths are, indeed, scattered at the surface, while the rest is transmitted. One thing to keep in mind is the fact that the blue is weakly absorbed in respect to the red wavelengths of light, which are strongly absorbed already in the first few meters of the water column.

So the red is “consumed” quickly, followed by orange, yellow and green, as you can see from here. The blue, however, is the only one that really penetrates into the depths of around -260 m (if the water is very clear). Eventually, the water molecules begin to vibrate in the blue, too, and so they absorb these wavelengths and the ocean becomes dark.

That is why the colour of the coastal sea might not be as navy blue as that of the open sea. Again, the coastal waters absorb mainly the wavelengths from red to green, whereas the open waters that reach higher depths transmit and lastly absorb a decent amount of the blue.

Just as the water molecules get excited once that are hit by the sunlight, the organic compounds and phytoplankton (planktonic plant organisms such as microalgae), too, absorb the light and change their molecular state from unexcited to excited. What does happen is that the electrons, which are negatively charged parts of atoms, jump from their normal energy orbital to a higher-energy orbital.

In the case of algae, the most abundant pigments that are found within their cells are the ones that produce oxygen – chlorophylls. The moment in which they are hit by photons, the electrons found in the photosystems of the chlorophylls jump from the ground state to the excited state, activating the process of photosynthesis.

This is an extremely good way to determine the health of the Earth’s marine ecosystems, since the photosynthesis can be determined based on the chlorophyll concentration, measured by satellites. The blue and red wavelengths, in fact, are absorbed by the chlorophyll pigments, while the green is poorly absorbed, resulting in the green appearance of the algae (same for the plants on land).

Now you might understand more in detail why our oceans are blue and also what happens to the sunlight. There is probably no need of explaining why the Sun’s energy is of such importance to the planet. Without it, there would be no photosynthesis and neither the oxygen that the entire world consumes.