Top of page

Everyday Mysteries

« Back to Physics page

Question Why is the ocean blue?


There are several theories:
– Blue wavelengths are absorbed the least by the deep ocean water and are scattered and reflected back to the observer’s eye
– Particles in the water may help to reflect blue light
– The ocean reflects the blue sky

Aerial shot of the clear blue ocean off the coast of Guam, 2017. USGS Multimedia Gallery

Most of the time the ocean appears to be blue because this is the color our eyes see. But the ocean can be many other colors depending upon particles in the water, the depth of the water, and the amount of skylight.

The colors we see depend upon the reflection of the visible wavelengths of light to our eyes. The website provides a good explanation of how we see color.External

Blue Ocean. Tadashi Ikai, 2015. Prints & Photographs Division, Library of Congress

Wavelengths of light pass through matter differently depending on the material’s composition. Blue wavelengths are transmitted to greater depths of the ocean, while red wavelengths are absorbed quickly. Water molecules scatter blue wavelengths by absorbing the light waves, and then rapidly re-emitting the light waves in different directions. That is why there are mostly blue wavelengths that are reflected back to our eyes.

A horse-eye jack shadow hunts along the side of whitespotted filefish in the Gulf of Mexico. Emma Hickerson, photographer. Flower Garden Banks National Marine Sanctuary, NOAA Photo Library

Sometimes oceans look green. This may be because there is an abundance of plant life or sediment from rivers that flow into the ocean. The blue light is absorbed more and the yellow pigments from plants mix with the blue light waves to produce the color green.

Sometimes parts of the oceans will look milky brown after a storm passes. This is because winds and currents associated with the storm churn up sand and sediment from the rivers that lead into the oceans.

The ocean may also reflect the blue sky. However this is prominent only at relatively low angles and when the water is smooth.

NASA Ocean Data Shows ‘Climate Dance’ of Plankton. 2017. NASA Goddard, NASA Image and Video Library

Published: 11/19/2019. Author: Science Reference Section, Library of Congress

Have a question? Ask a science librarian