Optical Properties

Have you ever wondered why the water is blue; or not blue in many cases? Or how you can see underwater in some water, like pools, but not in a lake, creek, pond or ocean?Or why, if you are on a dock sometimes you can see objects on the bottom and sometimes  you can't? It's all a result of hydrologic optics.

Optics is the part of physics that deals with light in the visable sprectrum. Light from the sun appears white to the human eye. However, white light is composed of an entire spectrum of colors, which is
illustrated by a rainblow or a prism.

                      
                                                                      Sunlight scattering through the leaves of tree.
 

Raindrops divide white light into individual wavelengths, each of which has a different energy content and a particular color. A rainbow is always in order with the reds on the outside and the blues and violets on the inside of the arc. There many more colors in between the main colors you see, because the light spectrum is a continum of colors, which when in combination look white.

Optics also deals with light in the invisible spectrum, infrared and ultraviolet light. Hydrologic optical principles determine the penetration of biologically damaging ultrviolet light underwater.


Understanding the principles of the visible light spectrum will help us explore how light behaves in water: hydrologic optics. Water itself and many of the contents of the water act on light entering the water, reducing different wavelengths by different degrees, which alters the color we see and the transparency, or the clarity, of the water. Color and clarity are the primary ways in which the public assess the health of a body of water. Furthermore, clarity determines the suitability of the water body as a habitat for both flora and fauna.

 

Optical properties of the water are measures which help us define what is happening to light in a particular water body. They are determined by pure water itself and the kinds and amounts of materials dissolved and suspended within it. Optical properties vary extensively depending upon the type and location of the water body.

For example, an estuary or coastal ecosystem is exposed to more human activity than the deep ocean. They are shallow compared to an ocean and receive both fresh and marine water. These physical and chemical differences cause the optical properties to be distinct. The distinct optical properties cause light to behave differently in each water body, leading to different colors and clarity in each water body type.

    The depth to which sunlight penetrates into the water determines the transparency of a water body. Light penetration is dictated by the composition of the water. Water itself strongly absorbs light in the red region of the light spectrum, which is why clean, clear waters are blue. Other components in the water that impacts its optical properties, include colored dissolved organic matter (CDOM), decaying organic matter, inorganic particulate matter, such as silt and clay, and phytoplankton.

These components progressively diminish the amount of light energy underwater with increasing depth. This process is called light attenuation and is caused by:

·         Absorption (which removes light energy)

·         Scatter (which changes the direction of the light path, thereby increasing the likelihood of absorption)  

 


 

In many aquatic environments in the United States diminished water quality has resulted in a reduction in the amount of light penetrating the water. This reduction has caused large-scale die off of submerged aquatic vegetation (SAV), an important part of the ecosystem. Scientists at SERC are studying the role of solar radiation in aquatic environments and the effects of water quality on the light available to the organisms that live there. Researcher Charles Gallegos has been examining how various components such as phytoplankton, particulate matter, and dissolved compounds diminish light from the sun as it travels through the water column. He is also investigating the factors that govern the interaction between these elements and light penetration and distribution. Through his work in the Chesapeake Bay Gallegos has developed a diagnostic tool that allows managers to establish water quality targets for meeting SAV growth criteria and to evaluate water quality against those criteria.

Understanding the factors that govern light penetration as it relates to water quality is providing managers with tools to improve water quality and restore the health of aquatic ecosystems.