BIOLOGY 457/657 PHYSIOLOGY OF MARINE & ESTUARINE ANIMALS. April 7, 2004 LIGHT IN AQUATIC ENVIRONMENTS. INTRODUCTION: THE DUAL NATURE OF LIGHT. Light has both wave and particle properties. Particle nature: Light exists in discrete units ( quanta or photons )
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April 7, 2004
LIGHT IN AQUATIC ENVIRONMENTS
Light has both wave and particle properties.
Particle nature: Light exists in discrete units (quanta or photons)
Wave nature: Light is characterized by its wavelength (λ). The energy content of each quantum is inversely proportional to λ and directly proportional to frequency (υ). Natural light consists of a mixture of photons of many wavelengths, which together form a spectrum. Other wave properties of light include its ability to be refracted and scattered.
Light & Vision: The spectral range of light useful for vision is from about 300 nm (deep ultraviolet, or UV) to about 750 nm (far red). “Visible light”, seen by humans, ranges from about 400 to 700 nm.
Light can be meassured in terms of its energycontent (e.g. watts) or in terms of its quantalcontent (e.g. photons s-1). Since photochemical processes that occur in living things, including vision and photosynthesis, depend on the absorption of photons, quantal units are preferred for most life-science applications.
Irradiance is the amount of light falling perpendicularly onto a surface. In quantal units, it is measured as:
Photons/unit area/unit time/nm,
e.g. 2.3 x 1014 photons cm-2 s-1 nm-1
Radiance is the amount of light reaching a dectector from a given direction in space, enclosed by a solid angle. In quantal units:
Photons/unit area/unit time/unit solid angle/nm,
e.g. 6.5 x 1014 photons cm-2 s-1 sterradian-1 nm-1
Each and every photon of light consists of an electromagnetic wave vibrating in a single plane, so each has a polarized electromagnetic field. Whenever the photons contained in light have vibrational planes that are not completely random, the light is said to be partiallypolarized. (This is the typical situation in nature, both in air and in water.) If all the photons in the light have parallel electric vectors (e-vectors), the light is fullypolarized. (This is unusual in natural light.)
(In this illustration, a single photon travels from left to right. The green curve represents the strength of the electric field, and the red curve represents the magnetic field.)
Whenever light passes from one medium to another, it is refracted according to Snell’sLaw.
n = sin(θi) , sosin(θr) = sin(θi) .
(the refractive index for water is ~1.33)
As a consequence, light entering water from air is confined to a conical overhead “window”, called Snell’sWindow, about 97° in diameter. Light from outside Snell’s window reaches a point in the water (for instance, an eye) by total internal reflection. The sharpness of the “edge” of Snell’s window depends on the flatness of the water’s surface.
Light is attenuated in water by absorption and by scattering. The amount by which light is attenuated is described by:
Iz(λ) = I (λ) e-k(λ) z ,
where I is intensity, z is depth, λ is wavelength, and k is the attenuation coefficient. Since attenuation varies with wavelength, the interaction of water with light can be described by an attenuation spectrum.
The major sources of attenuation are (1) absorption by water itself, (2) absorption by dissolved organic molecules (“gelbstoff”), (3) absorption by chlorphyll (in highly productive waters), and (4) scattering by suspended particles and sometimes planktonic organisms.
From Smith & Tyler (1972)
Pure water is blue, and since water of the open ocean is essentially pure (except for colorless dissolved inorganic salts), it transmits maximally in the blue. Coastal or estuarine waters are less transparent than ocean water, due to dissolved organics and suspended material, and transmit maximally at green or even yellow wavelengths.
Light also varies in spectral content depending on the time of day. Light at midday is almost white, which similar numbers of photons at all wavelengths. Light at twilight is enriched in both blue and red wavelengths. Moonlight is “warmer” than daylight, as the moon reflects few short-wavelength photons. Starlight tends to be greenish (not because stars are green, but because the upper atmosphere has a green auroral glow).
Due to waves and ripples at the surface of the water, light in water at a given depth can vary rapidly in intensity. The amount of flicker depends on the surface state of the water and the depth of the measurement.
Light in the atmosphere is polarized due to scattering by gas molecules and suspended particles, and some of this pattern passes into water through Snell’s window. Most polarization in water, however, arises from scattering within the water itself.
Adapted from Lythgoe (1988)
Light in water is scattered and attenuated both on its way to the object to be visualized, and on its way from this object to the viewer. This decreases contrast and becomes particularly problematical when light is low and the object reflects little light (as illustrated on the right).