Why seas are blue

When light shines through water, colors with longer wavelengths are absorbed by the water, with the longest wavelengths absorbed first.

As soon as we get more than a few meters underwater, most red and orange have vanished entirely. Next to go are yellow and green. Blue and violet, on the other hand, have the shortest wavelengths of visible light, so they are able to penetrate the deepest.

Other things can change the color of the water. Silt and sand may wash into the ocean from land. Or churning waves may lift them up from the sea floor. These sediments reflect more of the longer wavelengths of light, turning the water shades of brown. Water can also appear green—or red—in areas with lots of phytoplankton.

These tiny plant-like algae live near the surface, turning sunlight into energy that feeds the ocean ecosystem. They contain molecules like chlorophyll that absorb some wavelengths of light and reflect others. Shallow areas with clear water often appear turquoise. It bounces off the sandy bottom, which turns the water a brilliant blue. Extremely shallow areas still have some of the green wavelengths of light.

This creates the green-blue hues that we see in areas around islands and reefs, such as those in the Caribbean Sea. As a result, many deep-sea organisms are either black or red.

This is because a red surface absorbs blue light, effectively making these animals invisible under the tiny amount of light that filters down from above. At the same time, many organisms that live in the deep ocean have evolved the ability to produce their own light. This is known as bioluminescence. Organisms use it to communicate with other animals, find mates, and hunt for food. Braun C. Why is Water Blue? Journal of Chemical Education. Vol, 70, August Dickey, T. Shedding new light on light in the ocean.

The particles in these materials that are responsible for scattering the light are larger than the wavelength of light. Consequently, all colors of light are scattered by more or less the same amount. Much of the scattering in milk is due to the lipids fat. If you take out the fat, the milk will not scatter as much light; that is probably why skim milk looks the way it does. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue.

Some of the salt in the sea comes from undersea volcanoes and hydrothermal vents, but most of it comes from the land. Rain water dissolves minerals and releases salts from the rocks on land, which are then carried down to the sea by rivers. As the sun warms the sea, water evaporates but the salt is left behind, making the sea even saltier.

The saltiness called salinity of the sea is not the same in all parts of the world. Near the equator, where temperatures are higher, more evaporation takes place and so the seawater has a higher concentration of salt. Near the poles, melting ice and heavy rain dilute the seawater, making it less salty. The sea often appears blue because of the way light interacts with the water. It is exactly the same when it comes to light hitting water molecules: if there are more water molecules in the way, more light particles will collide with them and be scattered, making the water appear a deeper shade of blue.

There are of course other things that can change the colour of water. Lots of mud and sediment in rivers can make them appear brown; the same is true in shallow areas of the ocean, especially if a storm has stirred up lots of sediment from the seabed. Algae can turn the water green, because they contain the green pigment chlorophyll. A few species of algae may even turn the water red because they contain red pigments.

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