Physical Properties of Water and Light

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Physical Properties ofWater and Light

• Importance of water
• Water characteristics
• Nature of light

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Earth is the water planet!

• Water covers 70 of the earths surface
• 97.1 of it in the oceans
• 2.2 in polar ice caps and glaciers
• 2 in lakes and reservoirs
• Abundant in living organisms and life processes
• i.e. metabolic reactions, including
  photosynthesis
• The universal solvent

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Important Water Characteristics

• Polar molecule
• good solvent
• High specific heat
• good heat storage
• Hydrogen bonding
• high melting and boiling temperatures

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Characteristics, continued

• Viscosity
• Viscosity is a liquids resistance to flow
• It affects physical and biological processes
• Equilibrium concentrations of H3O (hydronium
  ions) and OH- (hydroxyl ions)
• 2H2O lt---gt H3O OH
• pH measurements, buffering

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Water density vs. Temperature
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Light A long journey

• Solar Spectrum

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Before entering the water, sunlight is

• Reflected by the atmosphere
• Absorbed and scattered by atmospheric particles
• Reflected by clouds
• Reflected by the water surface

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Upon entering the water, sunlight can be

• Scattered (water, suspended matter)
• Absorbed (water, dissolved materials, particulate
  matter, or bottom sediments)
• Backscattered to the surface

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Light attenuation depends onwavelength
Absorption by pure water
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Light changes with depth
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Attenuation the Lambert-Beer Law

• Iz I0 exp(-Kz) or -ln (Iz/I0) K z
• where
• z depth or distance m
• Iz light intensity at depth z
• I0 light intensity at the surface
• K attenuation coefficient m-1, which varies
• with wavelength, ?
• (K is also called e in the text)

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Lambert-Beer Math in action
Example Depth Surface 1 m 90 2
80 5 60 10 32 20 10 So, K for green
light (in this case) is approximately 0.11 /m
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Light Attenuation Case Study
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Light Attenuation Case Study
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Summary

• Physical characteristics of water fundamentally
  affect the chemistry and ecology of lakes
• Water is uniquely odd Hydrogen bonding, etc.
• Light in water behaves in interesting ways
• Attenuation may be constant, but light intensity
  drops exponentially
• By log-transforming an irradiance profile, we can
  derive the attenuation coefficients