For many years, people believed (and some still do!) that the ocean is blue because it reflects the blue sky. The real reason the ocean appears blue does involve reflection, but it’s not because the ocean acts as a mirror to the sky.
According to the National Oceanic and Atmospheric Administration, the ocean is blue because of how it absorbs sunlight. When sunlight hits an ocean’s waters, the water molecules naturally absorb the light rays.
Not all rays are absorbed in the same way, however. Water molecules mainly absorb longer wavelengths of light, such as those in the red, orange, yellow, and green wavelengths. Shorter wavelengths, like the blue wavelengths, tend to get reflected back to our eyes. This is why we usually see ocean water as blue.
Ocean water doesn’t always look blue, though. And sometimes it appears to be deeper blue than at other times. For example, the farther out in the ocean you go, the deeper the water gets. The more water there is, the more light gets absorbed. That’s why deep ocean waters may appear dark navy blue.
Shallow waters closer to shore may be other lighter shades of blue or even other colors, such as green. These colors may result from reflections of light bouncing off of the ocean floor, floating sediments, or plant life, like algae and phytoplankton.
You may also notice that other bodies of water, like rivers and ponds, appear to be colors other than blue. Some rivers, for example, may appear to be a muddy brown rather than a beautiful blue. The brown color of some bodies of water often results from the presence of sediments in the water, especially after the water has been stirred up by a storm or heavy rains.
If you’ve ever seen the crystal-clear waters of the Caribbean, which are usually shades of light green rather than clear, you may have WONDERed why those ocean waters are so much clearer than others.
Some people believe the Caribbean waters are cleaner and contain less pollution. That can certainly be true and have an impact. However, the colors are much more likely a result of the particular characteristics of the body of water in question.
For example, the plant life that exists in many Caribbean waters can contribute to more green light being reflected back. Some Caribbean waters that are exceptionally beautiful also tend to be more shallow. In addition, the composition of the ocean floor can have an effect. The ocean floor in the Caribbean often contains coral, which can reflect light differently than the sands found in many other areas.