• The bright blue of the Pacific, near the Galapagos Islands, above, fades to darkness as the ocean deepens. Light intensity fades for two reasons: absorption and scattering.

    Many substances absorb light in the ocean. Water itself absorbs light, mostly red light—which is why "clear" water appears blue. (The red light is absorbed, while the blue light is not. It is reflected.) Microscopic particles in the ocean, invisible to the human eye, also absorb light. Some of this microscopic material is organic, such as algae. Other microscopic materials are compounds released by chemical reactions in the ocean, such as the decay of plants.

    Light scatters as it hits microscopic particles and changes direction. Many of these particles are the same ones (algae and chemicals) that absorb light. Light is also scattered by sediment and other larger particles.

    Absorption and scattering darken the ocean to pitch-black at about 1,000 meters (3,280 feet) below the surface.

  • Term Part of Speech Definition Encyclopedic Entry
    absorb Verb

    to soak up.

    algae Plural Noun

    (singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.

    chemical reaction Noun

    process that involves a change in atoms, ions, or molecules of the substances (reagents) involved.

    decay Verb

    to rot or decompose.

    electromagnetic spectrum Noun

    continous band of all kinds of radiation (heat and light).

    microscopic Adjective

    very small.

    ocean Noun

    large body of salt water that covers most of the Earth.

    Encyclopedic Entry: ocean
    organic Adjective

    composed of living or once-living material.

    reflect Verb

    to rebound or return light from a surface.

    sediment Noun

    solid material transported and deposited by water, ice, and wind.

    Encyclopedic Entry: sediment
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