Why is water Blue?

Water is blue because of the way light is affected by water itself. In a glass or the bathtub, water appears clear, because it is being viewed over a short distance. However, this same water would look blue if a large volume of it was contained. When a body of water doesn't appear blue, it means that other constituents within the water are affecting its color.

To understand why water is blue we must understand light and how it behaves in water. Light is electromagnetic radiation from the sun in the color spectrum, which is the region of light to which the human eye is sensitive. The spectrum is a series of wavebands arranged by decreasing energy and assigned colors in the visible region. The human eye is sensitive to wavelengths from 400 nanometers (nm), which is blue to 700 nm, which is red with all the variations of both colors falling between.

Radiance is the amount of light energy recorded in a given time and area from a particular direction. It would seem that radiation would always be coming from the same direction, because it all originates from the sun; however, reflection and general light scatter cause light to come from all directions. Therefore, light underwater is measured by irradiance.

Irradiance is the radiance over all directions and is easily measured by a spherical probe. When measuring the light intensity underwater, it is useful to measure the downward irradiance and the upward irradiance. Downward irradiance is the radiance from the whole upper hemisphere, while upward irradiance is the radiance from the whole lower hemisphere. Downward irradiance captures all the light penetrating down into the water column, while upward irradiance captures all the backscattered and bottom reflected light, therefore also telling the overall beam attenuation coefficient.


Graphic depiction of wavelength of colored light on the x-axis, relative sensitivity of the human eye on the y-axis.  Most visible light peaks at 550 nanometers.


Above is the spectrum with the corresponding colors that we see. The graph on the top half of the diagram shows the sensitivity of the human eye. If you look at the highest point of the curve, at 1.0, it corresponds with wavelengths around 555 nm. Color is the result of the isolation just one or a few wavelengths from a ray of light. Isolation occurs when a substance absorbs particular wavelengths, leaving unabsorbed wavelengths. These unabsorbed wavelengths reflect color, which is detected by the human eye.

 Water is blue, because water itself absorbs light very weakly in the blue and green region of the spectrum, from below 400 nm to about 550 nm. However, at about the 550 nm, absorption begins to increase significantly into the red region of the spectrum, as can be seen in the graph below.



Most of the energy from the wavelengths in the green region above 550 nm, through the yellow and orange regions to the red wavelengths at 700 nm, is absorbed. Therefore only the blue and blue-green wavelengths remain significantly unabsorbed. The unabsorbed rays of light penetrate deeper into the water column than any of the absorbed wavelengths, meaning a blue color reflects back at you when you observe the water.

Have you ever seen green, yellow, or brown bodies of water? Pond waters can be green or yellowish, muddy waters look brown. There is a phenomenon called a red tide that can change the color of water to a reddish hue, from a red-orange to a milk chocolate or root beer color. It is caused by dinoflagellates that have a reddish pigment in them and when they bloom the water turns red.

              This photo represents a live sample of the dinoflagellate
             Prorocentrum minimum. The red pigment is visible because the slide is under a
             100x oil lens using a fluorescent lamp.

This is a salt marsh in Brewster, MA with Cape Cod Bay visable offshore. The water appears blue
because it is clear and the blue from the sky is (reflected or absorbed) by the water.


Water colors other than blue or blue-green occur because of particles of dissolved
substances, decaying matter and living organisms within the water.


Absorption is quantified by the absorption coefficient a, in units per meter. This can be directly measured for non-scattering samples, which are samples that do not contain substances that will scatter light. This is done using a spectrophotometer.

Total scatter is the combination of reflection, refraction and diffraction and is quantified by the scattering coefficient, b, in inverse meters. Scatter cannot be directly measured; rather, it is calculated using quantities that can be directly measured: absorption and beam attenuation.

The total scattering values tend to be relatively low in clean waters and rise with increased concentrations of particulate matter. When normalized to the total scattering coefficient, however, the angular distribution of scattering tends to be similar in diverse bodies of water.

Beam attenuation is quantified by beam attenuation, c, in inverse meters, when measuring inherent optical properties.

A second type of absorption is inelastic, unbendable scattering, or fluorescence. It is not considered as important as absorption or scattering, because it only occasionally affects water color. Its main effect is in conjunction with chlorophyll from phytoplankton at 680 nm. During fluorescence a photon is absorbed by a particle, but it only loses part of its energy content. The remaining energy is re-radiated in all directions from the particle.

                               APPARENT OPTICAL PROPERTIES OF WATER

Apparent optical properties are those characteristics of the water body that are dependent on the ambient light; therefore, the measurements cannot be taken in the laboratory, only in situ (in the field). Apparent optical properties are determined by:

  • Secchi disk depth
  • Irradiance attenuation 


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