Why the sea water is in blue?

         Water is transparent, but the sea looks blue to us. This is not because it reflects the color of the sky, but because of the composition of sunlight, which is made up of the familiar colors of the rainbow. When it strikes the surface of the water, the water absorbs all of the colors except for blue-the blue is reflected,  and this is what our eyes see.

Seas are not always blue, however. Other colors occur as a result of the various substances that can be bound in sea water. Microscopic algae give the water a green hue, and if a large quantity of algae are present as they are in the Baltic sea in summer-the water turns a greenish-brown. The chlorophyll contained in some blue-green algae colors the water red, and yellow is seen when there are large concentrations of sand and clay particles in the water, such as in the Yellow Sea between China and the Korean peninsula.