why seawater is blue From: Encyclopedia of Earth Science.
At some point nearly every child with an inquisitive mind will ask an adult why the sky is blue or why the seawater is blue. In a simple sense, seawater is blue because is it a reflection of the color of the sky above, but then why is the sky blue? The answer lies in phenomena called scattering of light. Sunlight that enters the atmosphere contains the complete visible spectrum of colors, defined by different wavelengths. As this light enters the atmosphere it encounters air molecules of oxygen and nitrogen, each of which is smaller than the wavelength of visible light. These molecules cause the incident light to be scattered when the light hits them, but since the molecules are small they are much more effective at scattering the short wavelengths than the long wavelengths, which tend to pass over the small molecules. This selective scattering is analogous to ocean waves that encounter a buoy in the water. Waves that are small (short wavelength) and about the same size as the buoy will bounce off and be scattered by the buoy, but large waves (long wavelength) will pass right by the buoy and hardly be affected. Similarly, the shorter visible wavelengths of light, including violet, blue, and green, are scattered efficiently by the small air molecules, whereas the longer wavelengths of yellow, orange, and red are scattered very little. The atmosphere scatters blue light about 16 times as much as red light. The result of this scattering is that as we look at the sky, we see blue light coming at us from virtually all directions. In contrast, the yellow, orange, and red light are not scattered effectively and appear to be only coming nearly directly from the direction of the Sun. The same effect causes distant mountains to appear blue, when scattering by small particles is strong near the ground. The presence of larger particles can cause different color sensations. For instance, the presence of larger aerosol pollutants causes a brownish smog color, and the presence of even larger water droplets causes clouds and haze to appear white. Most of the light and energy from the Sun that strike the sea is absorbed by seawater and converted to heat, but some is reflected. The upper surface of the sea reflects the color of the sky, which is most often blue. However, the presence of suspended particles in seawater can further alter the color of light perceived in the water. For instance, clear ocean waters appear deep blue or violet, whereas coastal waters with large amounts of suspended sediments or dissolved organic substances causes the reflected light to shift to longer wavelength colors such as green. In turbid coastal waters the shift in the wavelength of reflected light is enough to change the color to yellow.
Citation Information Record Citation: Kusky, Timothy. "why seawater is blue." Encyclopedia of Earth Science. New York: Facts On File, Inc., 2005. Science Online. Facts On File, Inc. http://www.fofweb.com/activelink2.asp?ItemID=WE40&SID=5&iPin=EOES0614&SingleReco rd=True (accessed February 24, 2010).
Record URL: http://www.fofweb.com/activelink2.asp?ItemID=WE40&SID=5&iPin=EOES0614&SingleReco rd=True.