question: Why do large amounts of clear water look blue?
comment: The water is blue as red, yellow and orange wavelengths of light are absorbed much more than blue wavelength by water. So, the blue wavelength of light is mostly reflected back, and this light reaches our eyes, giving the sensation of clear blue water. Hope I could help ;)
passages: Scattering from suspended particles also plays an important role in the color of lakes and oceans. A few tens of meters of water will absorb all light, so without scattering, all bodies of water would appear black. Because most lakes and oceans contain suspended living matter and mineral particles, known as colored dissolved organic matter (CDOM), light from above is reflected upwards. Scattering from suspended particles would normally give a white color, as with snow, but because the light first passes through many meters of blue-colored liquid, the scattered light appears blue. In extremely pure water—as is found in mountain lakes, where scattering from white-colored particles is missing—the scattering from water molecules themselves also contributes a blue color.
   Absorption intensity decreases markedly with each successive overtone, resulting in very weak absorption for the third overtone. For this reason, the pipe needs to have a length of a meter or more and the water must be purified by microfiltration to remove any particles that could produce Mie scattering.
  
  Section::::Color of lakes and oceans.
  
  Lakes and oceans appear blue for several reasons. One is that the surface of the water reflects the color of the sky. While this reflection contributes to the observed color, it is not the sole reason.
   Some constituents of sea water can influence the shade of blue of the ocean. This is why it can look greener or bluer in different areas. Water in swimming pools (which may also contain various chemicals) with white-painted sides and bottom will appear as a turquoise blue.
  
  Clean or uncontaminated water appears blue in white-tiled swimming pools as well as in indoor pools where there is no blue sky to be reflected. The deeper the pool, the bluer the water.
   The intrinsic colour of liquid water may be demonstrated by looking at a white light source through a long pipe that is filled with purified water and closed at both ends with a transparent window. The light turquoise blue colour is caused by weak absorption in the red part of the visible spectrum.
   Section::::Mechanism of colour change.
   Relatively small amounts of regular ice appear white because plenty of air bubbles are present, and also because small quantities of water appear to be colorless. In glaciers, on the other hand, the pressure causes the air bubbles, trapped in the accumulated snow, to be squeezed out increasing the density of the created ice. Large quantities of water appear blue, therefore a large piece of compressed ice, or a glacier, would also appear blue.
  
  Section::::Color of water samples.
   Section::::Sedimentation and blue carbon burial.:Spatial variability in sedimentation.:Submarine Canyons.
   The "color" of the ocean is determined by the interactions of incident light with substances or particles present in the water. White light from the sun is made up of a combination of colors that are broken apart by water droplets in a "rainbow" spectrum. Large quantities of water, even in a swimming pool, would appear blue as well. When light hits the water surface, the different colors are absorbed, transmitted, scattered, or reflected in differing intensities by water molecules and other so-called optically-active constituents in suspension in the upper layer of the ocean. The reason that open ocean waters often appear blue is due to the absorption and scattering of light. The blue wavelengths of light are scattered, similar to the scattering of blue light in the sky but absorption is a much larger factor than scattering for the clear ocean water. In water, absorption is strong in the red and weak in the blue and so red light is absorbed quickly in the ocean leaving blue. Almost all sunlight that enters the ocean is absorbed, except very close to the coast. The red, yellow, and green wavelengths of sunlight are absorbed by water molecules in the ocean. When sunlight hits the ocean, some of the light is reflected back directly, but most of it penetrates the ocean surface and interacts with the water molecules that it encounters. The red, orange, yellow, and green wavelengths of light are absorbed and so the remaining light we see is composed of the shorter wavelength blues and violets.
   The sea is seen as blue for largely the same reason: the water absorbs the longer wavelengths of red and reflects and scatters the blue, which comes to the eye of the viewer. The colour of the sea is also affected by the colour of the sky, reflected by particles in the water; and by algae and plant life in the water, which can make it look green; or by sediment, which can make it look brown.
  
  Section::::Science and nature.:Atmospheric perspective.
   Section::::Composition.
   Color of water
  
  The color of water varies with the ambient conditions in which that water is present. While relatively small quantities of water appear to be colorless, pure water has a slight blue color that becomes a deeper blue as the thickness of the observed sample increases. The blue hue of water is an intrinsic property and is caused by selective absorption and scattering of white light. Dissolved elements or suspended impurities may give water a different color.
  
  Section::::Intrinsic colour.
   While monochromatic photos make this effect appear white, Monterey Bay Aquarium Research Institute scientist Steven Haddock (an author of a milky seas effect study) has commented, "the light produced by the bacteria is actually blue, not white. It is white in the graphic because of the monochromatic sensor we used, and it can appear white to the eye because the rods in our eye (used for night vision) don't discriminate color." In Shetland (where generally caused by "Noctiluca scintillans"), mareel has sometimes been described as being green, rather than the traditional blue or white milky seas effect seen by the rest of the world. It is not known whether this difference depends on the area, or simply a perception of a cyanic colour as being green.
   BULLET::::- Color spectrum and water
  
  Light is absorbed and reflected in water. Visible white light is made up of a spectrum of colors from the rainbow, ranging from red to violet. As the light travels through the water, the waves of light from the red end of the spectrum dissipate (i.e. are absorbed), while those from the blue end, become more prominent.
  
  Section::::RMS "Titanic".
   The title 'blue' arose because there are minute particles of clay in colloidal suspension within the water. These suspensions variously diffract the light depending on the particles' size (with differences in size often linked to changes in concentration and pH), yielding colours from red-brown, through grey and green, to the more typical turquoise. The metallic blue sometimes found in photographs is more likely to be a reflection of the sky, as the attraction's own website makes no such claim for the water, and their leaflets call it “a turquoise jewel set in the heart of Purbeck”.
   Section::::Dye.
  
  Bromophenol blue is also used as a dye. At neutral pH, the dye absorbs red light most strongly and transmits blue light. Solutions of the dye, therefore, are blue. At low pH, the dye absorbs ultraviolet and blue light most strongly and appears yellow in solution. 
   Section::::Background.
   Various cultures divide the semantic field of colors differently from the English language usage and some do not distinguish between blue and green in the same way. An example is Welsh where "glas" can mean blue or green.
  
  Other color names assigned to bodies of water are sea green and ultramarine blue. Unusual oceanic colorings have given rise to the terms red tide and black tide.
   In clear open waters, visible light is absorbed at the longest wavelengths first. Thus, red, orange, and yellow wavelengths are absorbed at higher water depths, and blue and violet wavelengths reach the deepest in the water column. Because the blue and violet wavelengths are absorbed last compared to the other wavelengths, open ocean waters appear deep-blue to the eye.
   The most literal meaning of "blue moon" is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere, as has happened after forest fires in Sweden and Canada in 1950 and 1951, and after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years. Other less potent volcanoes have also turned the moon blue. People saw blue moons in 1983 after the eruption of the El Chichón volcano in Mexico, and there are reports of blue moons caused by Mount St. Helens in 1980 and Mount Pinatubo in 1991. In the Antarctic diary of Robert Falcon Scott for July 11, 1911 his entry says, "... the air thick with snow, and the moon a vague blue". On that date the moon phase would have looked full.
   The colors below are arranged according to value (brightness) (the v code in hsv), lightest at the top and darkest towards the bottom.
  
  Section::::Variations of blue-gray.:Iceberg.
  
  The color iceberg is displayed at right.
  
  Although icebergs are white, from a distance they can often appear blue when the colors of the sky and the ocean are reflected off of them.
  
  The first recorded use of "iceberg" as a color name in English was in 1921.
  
  Section::::Variations of blue-gray.:Slate blue.
  
  Displayed at right is the web color slate blue.
   Icebergs may also appear blue due to light refraction and age. Older icebergs reveal vivid hues of green and blue, resulting from a high concentration of color, microorganisms, and compacted ice. One of the better known blue icebergs rests in the waters off Sermilik fjord near Greenland. It is described as an electric blue iceberg and is known to locals as "blue diamond".
  
  Section::::Physics of light and color.
  
  BULLET::::- White icebergs
   BULLET::::- 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 ground rock, such as glacial flour, are turquoise. 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.
  
  Section::::Color names.
   Section::::Color of glaciers.
  
  Glaciers are large bodies of ice and snow formed in cold climates by processes involving the compaction of fallen snow. While snowy glaciers appear white from a distance, up close and when shielded from direct ambient light, glaciers usually appear a deep blue due to the long path lengths of the internal reflected light.
   BULLET::::- 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.
   Blue Sea Lake
  
  Blue Sea Lake (in French: Lac Blue Sea) is a lake in the municipalities of Blue Sea and Messines, Quebec, Canada, about north of Gatineau. It is known for its crystal clear water and is surrounded by cottages on its shores.
  
  The blue waters of Blue Sea Lake, together with its dimensions and the absence of significant relief around, create the illusion of a sea. It feeds the Blue Sea River which empties into the Picanoc River, in turn a tributary of the Gatineau River. Its name first appeared on a map of Hull County in 1928.
  