Short answer: the ocean is blue mainly because water interacts with different wavelengths of sunlight differently. Red wavelengths are absorbed more strongly, while more blue light remains available to scatter back toward your eyes.
The sky helps sometimes—but it is not the main explanation
A smooth water surface can reflect the sky, especially at shallow viewing angles. But NOAA explains that the bulk color of clear ocean water comes from sunlight being filtered by the water itself.
Real oceans are not chemically or optically pure. Sediments, dissolved material and microscopic organisms can shift the color toward green, brown, turquoise or even red. Depth and viewing geometry also matter.
Quick follow-ups
FAQ
Is the ocean blue only because it reflects the sky?
No. Reflection can contribute, but water’s wavelength-dependent absorption and scattering are central to the ocean’s blue appearance.
Why is some ocean water green or brown?
Particles, sediments and biological material change which wavelengths are absorbed and scattered back to the observer.
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