The Real Reason The Ocean Is (Mostly) Blue

Covering more than 70% of the planet's surface, oceans have earned Earth the nickname "the Blue Planet," a moniker that inspired the acclaimed BBC documentary series of the same name. But why not Yellow Planet or Purple Planet or Burnt Ochre Planet? Why, of all the colors out there, are our oceans blue? The answer lies in the physics of visible light waves, specifically, how the ocean absorbs and reflects different colors of light.

The color of any object is determined by which wavelengths of visible light that object reflects, and which ones it absorbs. Water happens to absorb light waves in the range of 600 nanometers to 800 nanometers, which covers orange and red light. Therefore, when sunlight hits the ocean, all of the warm color gets absorbed, and shades of blue, which have much shorter wavelengths in the range of 450 to 500 nanometers, get reflected back for our eyes to see.

Although we see the ocean as blue from our position above the waves, it isn't an accurate reflection of the true color of the sea. In reality, most of the ocean has no color at all. Although blue light waves can penetrate deeper than the longer light waves of warmer colors, little light falls beyond 656 feet below sea level. Beyond 3,280 feet below sea level, no light shines at all, and yet the average depth of the ocean is around 12,000 feet, meaning most of the water is a pitch black void.

The color of the ocean varies between ocean ecosystems. The open ocean is much darker than coastal waters, while some tropical regions, most notably around the Bahamas, are a brilliant turquoise hue. These varying shades of blue are caused by the depth of the water. In shallow areas closer to land, light penetrates all the way through the water to the seafloor and is reflected back, but in the open ocean, light gets swallowed up long before it touches the bottom. In the case of the Bahamas and similar tropical areas, the seabed is covered in white sand, which causes a very bright reflection.

Materials in the ocean water also affect its color. Green waters signal the presence of algae and other aquatic plants. Plants absorb warm colors of light, just as water does, but reflect back green wavelengths more than blue. Certain types of phytoplankton even produce toxins that color the water red, a so-called "red tide." Other parts of the ocean may appear to have a muddy brown or grey color, which is caused by high levels of sediment in the water. This tends to happen in the aftermath of storms, which can whip up materials from the ocean floor.

There is a popular myth that the sky is blue because it reflects the ocean, but this is false. The atmosphere scatters blue light waves more than red light waves, and it is actually the ocean that reflects the sky, amplifying its blue color.