Why Is the Ocean Blue? How Water Filters Sunlight, per NOAA
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How we wrote this: A plain-English summary of NOAA's National Ocean Service page "Why is the ocean blue?", checked October 29, 2026.
Water is a light filter
Sunlight contains every color. According to NOAA, water absorbs colors in the red part of the spectrum. Like a filter, it leaves the blue part for us to see.
Why it isn't always blue
The ocean can look green, red or other colors when light bounces off floating sediment and particles. Near the shore, stirred-up sand, plankton or algae can change the color you see from the beach (see seaweed and algae).
Most of the ocean is dark
- Hardly any light gets deeper than 656 feet (200 meters).
- No light reaches below 3,280 feet (1,000 meters).
At the beach
The color of the water says nothing about whether it's safe to swim. Check posted beach advisories, rip current warnings and the lifeguards; see rip current safety.
Related: why the ocean is salty, what is sea foam.