Muddy Creek, a tributary of the Chesapeake Bay, around noon
Have you ever wondered why 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 the ocean? Or why, if you're 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 visible spectrum. 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 rainbow or a prism.
Raindrops divide white light into individual wavelengths, each of which has a different energy content and a particular color. A rainbow is always ordered with the reds on the outside and the blues and violets on the inside of the arc. There are many more colors in between the main colors you see, because the light spectrum is a continuum of colors, which when in combination look white.
Optics also deals with light in the invisible spectrum, infrared and ultraviolet light. Hydrologic optical principles determines the penetration of biologically damaging ultraviolet 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 impact its optical properties, include colored dissolved organic matter (CDOM), decaying organic matter, inorganic particulate matter, such as silt and clay, and phytoplankton.
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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)
The diagram below shows how each of the major components within the water affect light attenuation.
When measuring the light intensity in water, the reduction in light energy with depth is mostly the result of absorption. The molecules that make up water, particulate matter and dissolved substances within the water can convert the photons, which make-up the light energy within a light beam, into a non-radiant form when the photons come into contact which the substance. This non-radiant form is usually given off as heat. As a result of absorption, fewer photons, therefore less light, are penetrating further downward into the water column.
The intensity with which a substance absorbs light depends on the wavelength of the light. It is the wavelengths that penetrate deepest that determine the color of the water. For example, water molecules selectively absorb energy from lower energy wavebands in the reds, yellows and greens, leaving only blue light. Therefore very pure and deep waters appear blue.
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Scattering also contributes to the attenuation of light whenever there are particles present in the water. Scattering causes underwater to appear fuzzy. This is because the light illuminating the object is being scattered away from its edges, causing the distinct outlines of the object to fade.
Particles can scatter light in all directions, though most scattering occurs in the forward direction. Light scattered backwards, towards the surface, is called backscatter. Backscatter reduces the amount of light traveling downward, but increases the apparent brightness to an observer above the water. The diagram below depicts several ways that particles can scatter light.
Both reflection and refraction occur at the interface of two different media, such as at the surface of the water (the air/water interface), or at the surface of a particle. Reflected light bounces off the new medium, i.e., the water's or a particle's surface.
Refraction is the result of a change in the velocity of light, which occurs when light enters a new medium. Light travels faster through air than through water, glass or a particle. As a result of the change in speed, the direction of travel will change, as can be seen in the diagram. The light changes direction whenever there is a change of medium, both when it enters the particle and when in leaves the particle.
Diffraction is a change in the direction of the light, due to the proximity of the particle. As a beam of light approaches a particle, the effect of the particle on water causes the light to change directions. Diffraction affects as much light as a large particle itself absorbs and scatters, doubling the amount of attenuated light by one particle (Davies-Colley et al., 1993).
Scatter also leads to FURTHER absorption. Changing directions increases the pathlength of the light, thereby increasing its probability of being absorbed.
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Inherent optical properties of water
Inherent optical properties (IOP) are those characteristics of water which depend only on the content of the water. In principle, IOPs can be measured in the laboratory, at night or at depths too great for light to reach. That is, inherent optical properties do not depend on the presence of sunlight under water.
Four properties make up the IOPs:
- beam attenuation
- the volume scattering function
Beam attenuation is a measure of the rate of decrease of a "collimated" beam of light (one which has parallel waves of light) through a given pathlength. Below is a diagram illustrating how beam attenuation is measured. Light is focused into a parallel beam, and passed through a tunnel containing a water sample. The reduction in light from the beam is measured at the end of the tunnel. Beam attenuation is the sum of absorption and scatter. The lower the attenuation, the more light is penetrating into the depths of a water column.
The volume scattering function accounts for the new direction of scattered light. It incorporates the angle of scattering with respect to the incident beam of light. The function may be thought of as the probability of light being scattered in a particular direction relative to its initial direction. For example, water molecules and dissolved substances are likely to scatter light in any direction, so that the volume scattering function is nearly the same for all directions. Particulate matter, especially phytoplankton, scatters most of its impinging light in the forward direction (towards the bottom). This is particularly a result of diffraction. The volume scattering function is therefore dependent on the composition of the water, as with all the IOPs.
Apparent optical properties of water
Apparent optical properties are the measures of light penetration in water that are dependent on the sunlight as well as the IOPs; therefore, all measurements of AOPs must be taken in situ, or in the field. These properties will vary slightly with the time of day the measurements are taken, because they are dependent on the intensity and angle of the sun. They cannot be recorded at night or at depths below which sunlight penetrates.
Taking AOP measurements
The time of day is important because it determines the angle of the rays of light from the sun to the water. At noon, when the sun is at its highest, more light penetrates into the water column than at 6:00 in the morning or evening, when reflection is greater. Other factors, such as cloud cover and surface waves, affect the intensity of the light underwater, and therefore also the measurement of AOPs.
Some apparent optical properties are:
- Secchi disk depth, zSD
- Irradiance attenuation coefficient
- Irradiance reflection coefficient
Secchi disk depth is a visual measure of the transparency of the water. It is dependent on the ambient light field because the more light that is available the deeper the human eye will be able to detect the disk. Because it depends on a human observer, it is especially sensitive to scattering, which degrades the image of the disk as it is lowered.
A Secchi disk emerging from the water.
The irradiance attenuation coefficient is an estimate of the amount of light which is attenuated by absorption or scattering, over an entire water column. It is directly measured over small depth intervals, using light sensors that are lowered underwater, as shown above.
The irradiance reflection coefficient is an indicator of scatter and relates to the brightness of the water. The surface reflectance, which is measured just below the surface of the water, is an important estimator of brightness.
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