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Why does the sea looks green sometimes?
There are a few reasons why the ocean is blue. The best answer is that the ocean is blue because it is mostly water, which is blue in large quantities. When light strikes water, like sunlight, the water filters the light so that red is absorbed and some blue is reflected. Blue also travels further through water than light with longer wavelengths (red, yellow, green) though very little light reaches deeper than 200 meters  and no light at all penetrates beyond 2,000 meters .

Another reason the ocean appears blue is because it reflects the color of the sky. Tiny particles in the ocean act as reflective mirrors so a large part of the color you see depends on what is around the ocean.

Sometimes the ocean appears other colors besides blue. For example, the Atlantic off the East Coast of the United States usually appears green. This is due to the presence of algae and plant life. The ocean may appear gray under a cloudy sky or brown when the water contains a lot of sediment, as when a river empties into the sea or after the water has been stirred up by a storm.


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