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    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

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 Franklin Institute provides a good explanation of how we see color at

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 reemitting the light waves in different directions. That is why there are mostly blue wavelengths that are reflected back to our eyes.

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.

Standard DisclaimerRelated Web Sites
  • Centers for Ocean Sciences Education and Excellence - The Centers for Ocean Sciences Education Excellence (COSEE) is a network of seven regional centers that promote ocean science education. There is a link to the US Commission of Ocean Policy from the homepage. Also of interest are resources for teachers and students from the specific regional centers.
  • Molecular Expressions. Science, Optics and You: Light and Color - “The light and color section of Molecular Expressions Science, Optics & You explores many aspects of visible light, beginning with an introduction to electromagnetic radiation and continuing through to the perception of color and the characteristics of lenses.”
  • NOAA’s National Ocean Service (NOS) - “ NOS measures and predicts coastal and ocean phenomena, protects large areas of the oceans, works to ensure safe navigation, and serves the American public in many other ways… Under Topics visitors can read descriptions about NOS's primary roles, responsibilities and activities. The descriptions of these topics include links to other Web sites, primarily within NOS, should visitors want more specific or in-depth information. Under Education students and teachers can access online discovery kits. The kits cover ocean-related science topics related to the work of NOS and include tutorials with illustrative graphics, extensive resource lists, and lesson plans .”
  • Ocean Exploration - Ocean Exploration “is an educational Internet offering for all who wish to learn about, discover, and virtually explore the ocean realm. It provides public access to current information on a series of NOAA scientific and educational explorations and activities in the marine environment. The site provides a platform to follow explorations in near real-time, learn about exploration technologies, observe remote marine flora and fauna in the colorful multimedia gallery, read about NOAA’s 200-year history of ocean exploration, and discover additional NOAA resources in a virtual library.”
  • UN Atlas of the Oceans - “The UN Atlas of the Oceans is an Internet portal providing information relevant to the sustainable development of the oceans. It is designed for policy-makers who need to become familiar with ocean issues and for scientists, students and resource managers who need access to databases and approaches to sustainability. The UN Atlas can also provide the ocean industry and stakeholders with pertinent information on ocean matters.”
    The About the Oceans section “provides an encyclopedic collection of information about the oceans.”
  • Why is Water Blue? - From the Causes of Color Web site, the section on “Why water is blue?” discusses how the blue color originates from vibrational transitions.

Library of Congress Web SiteFurther Reading
  • Falk, David S., Dieter R. Brill, and David G. Stork. Seeing the light: optics in nature, photography, color, vision, and holography. New York, Harper & Row, c1986. 446 p.
  • Littlefield, Cindy A. Awesome ocean science! Investigating the secrets of the underwater world. Charlotte, Vt, Williamson Pub., c2003. 120 p. (juvenile literature)
  • Lynch, David K. and William Livingston. Color and light in nature. 2nd edition. Cambridge, UK, New York, Cambridge University Press, 2001. 277 p.
  • Marine science on file. New York, Facts on File, c2001. 1 v. (various pagings).
  • Nye, Bill. Bill Nye the science guy’s big blue ocean. New York, Hyperion Books for Children, 1999. 48 p. (juvenile literature)
  • Rea, Thelma. I wonder why the sky is blue. New York, Rosen Pub. Group, 2002. 24 p. (juvenile literature)
  • Svarney, Thomas E., and Patricia L. Barnes-Svarney. The handy ocean answer book. Detroit, Visible Ink Press, c2000. 570 p.
  • Sverdrup, Keith A., Alyn C. Duxbury, and Alison B. Duxbury. An introduction to the world’s oceans. 8th edition. Boston, McGraw-Hill Higher Education, c2005. 514 p.
  • Waldman, Gary. Introduction to light: the physics of light, vision, and color. Mineola, New York, Dover Publications, c2002. 228 p.

SearchFor more print resources...
Search on "Light", "Light scattering", "Ocean" or " Oceanography" in the Library of Congress Online Catalog.

Coral Reef From NOAA National Ocean Service

Satellite image of Africa and surrounding waters. Global Biosphere, from the NASA SVS Screen Shots Web page

Graphic showing the seven colors off the visual spectrum. From NASA Web page Basic CCDs

Heron landing on a boat surrounded by blue water.
A lost heron hitching a ride at sea. From the NOAA Photo Library

Print - Graphic of a pelican flying over the ocean.
Blue Ocean. Tadashi Ikai. Donted by the artist to the Library of Congress.

Sea turtle. NOAA's Coral Kingdom Collection.

Photo of green water and trees on shore.
Green water in the Potomac River caused by an algae bloom. From NOAA Web site.

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 July 31, 2017
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