Why Is the Ocean Blue?

Have you ever wondered why the ocean is blue or why it is sometimes another color, like green, instead? Here's the science behind the color of the sea.​

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 farther through water than light with longer wavelengths (red, yellow, and green), though very little light reaches deeper than 200 meters (656 feet), and no light at all penetrates beyond 2,000 meters (6,562 feet).

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.

For more on the color blue in science, check out these articles:

• "Why Blood Isn't Blue"
• "Why Babies Have Blue Eyes"
• "Why Veins Look Blue"
• "Why Is Ice Blue"