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 constitutes within the water are affecting it's 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 visible spectrum, which is the region of the light spectrum 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 the higher energy wavebands) to 700 nm (lower energy wavebands).

Radiance is the amount of light energy recorded in a given time and area from a particular direction. It seems 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.

The irradiance attenuation coefficient, K, is the major apparent optical property. It can be directly measured and used to estimate the absorption coefficient and the scattering coefficient, therefore also telling the overall beam attenuation coefficient.

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, though not all, 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.

But you have seen green, yellow and brown water bodies??!! (Probably not red or orange, although in a few cases that is possible! See red tides in Explore Phytoplankton.) Well, water colors other than blue or blue-green occur because of particles, dissolved substances and living organisms within the water.

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

• Secchi disk depth
• Irradiance attenuation 

But what does this mean??

Photosynthetically available light is called photosynthetically active radiation (PAR) and it is a smaller amount of light than the total light recorded at any point.

Back to top