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The Mediterranean Sea off the coast of Southwest Spain. The sea is vibrant blue because the intrinsic blue color of water is reflected back from the light-colored sand.
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Why is water blue?
Water is faint blue. Although water appears clear in small quanities (like a glass of water), the blue color becomes visible the more water we look through. Thus, deep lakes and seas are bluer than a shallow river.
Other factors can affect the color we see:
- Particles and solutes can absorb light, as in tea or coffee. Green algae in rivers and streams often lend a blue-green color. The red sea has occasional blooms of red Trichodesmium erythraeum algae.
- Particles in water can scatter light. The Colorado river is often muddy red because of suspended reddish silt in the water. Some mountain lakes and streams with finely gound rock, such as glacial flour, are tourquise. Light scattering by suspended matter is required in order that the blue light produced by water's absorption can return to the surface and be observed. Such scattering can also shift the spectrum of the emerging photons toward the green, a color often seen when water laden with suspended particles is observed.
- The surface of seas and lakes often reflect blue skylight, making them appear bluer. [[[ Montana reflection.]]] The relative contribution of reflected skylight and the light scattered back from the depths is strongly dependent on observation angle.
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