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.

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.

Noun

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

organic
Adjective

composed of living or once-living material.

reflect
Verb

to rebound or return light from a surface.

Noun

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