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Oceans, seas and lakes come in various shades of blue or even completely different colors. The oceans appear to us blue due to a combination of several factors. We will discuss the main three:

Different substances absorb and reflect light waves of different frequencies
Visible light is a part of the electromagnetic radiation spectrum, ranging from red to violet. When light strikes a watery surface it penetrates it. Blue waves penetrate deeper than other waves in the visible spectrum, which are absorbed faster by the water molecules. Since blue wavelengths penetrate deeper, when the light is reflected back, blue is the dominant color.
 
Particles in the water reflect different light waves
Minerals and other particles reflect blue light waves in the oceans. Some other components of the oceans, mainly plants and algae, reflect light of other colors.
The Red Sea, for example, obtained its name from the seasonal blooms of red algae. Other bodies of water appear green due to green plants living in the water. In rivers, different sediments are carried to the surface, giving river water different colors.
At times, especially following a storm, oceans can appear brownish since the currents carry with them large amounts of sand. Shallow coastal waters tend to include large amounts of particles that reflect or absorb at different wavelengths. For this reason coastal waters often appear greenish or brown.
 
The water layer reflects light that is blue to begin with.
Another factor that gives water its color is the blue color of the sky. Since the sky "generates" a lot of blue light, much of it is reflected from the top water layer. Just like we can watch our reflection in still water, this reflective property also applies to the sky, hence giving the water a bluish tint.

 
Haim Barak
Department of Particle Physics and Astrophysics
Weizmann Institute of Science
 
English translation:
Dan Blat
Department of Immunology
Weizmann Institute of Science
 
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