Lecture 4: Light Flashcards
Planck’s Equation
e=hc/λ h is Planck’s constant, (6.63 x 10-34 J/s and λ is the wavelength of the photon (nm, 10-9m). Radiant energy is transported by photons which behave both as particles and waves and travels at the speed of light (c = 3 x 1010 m/s in vacuum). The photon energy of the radiation (e) in inversely related to the wavelength of the radiation, as described by Planck’s equation.
Variation in Light Intensity, Duration, and Directionality (Slide)
Units - Light is transported in discrete bundles called PHOTONS -Mole of Photons (6.02x1023) = 1 Einstein (E) -Einsteins are only used for PAR (400-740 nm) -The intensity of PAR is determined by the photon flux density, which is about 1500μE m-2 sec-1 near midday on a clear day
Lambert’s Cosine Law
Light intensity (LI) at the ground is influenced by its angle of incidence, which depends on the relative height of the sun above the horizon
Lambert’s Cosine Law
Light intensity (LI) at the ground is influenced by its angle of incidence, which depends on the relative height of the sun above the horizon
Lambert’s Cosine Law Equation
Lambert’s Cosine Law Diagram
Light Intensity Varies With…
Latitude
Season
Time of Day
Aspect
Slope
Albedo
Percent of incident solar radiation that is reflected from a surface
Leaf Area Index
Total leaf area per unit of ground area
Leaf Area Index (LAI) Equation
LAI= Total Lead Area/ projected ground area
Beer’s Law
Describes the relationship between leaf area index (L) and available light
Beer’s Law Equation
I=I0e-kLi
I= Available light at any height
e= base of natural logarithm (2.718)
Li= cumulative LAI above height i
k= Light extinction coefficient (measure of degree to which leaves absorb and reflect light)
Higher LAI and higher k → less available light
Euphotic Zone
Water depth where light intensity is above the compensation point (high enough to support photosynthetic organisms).
Compensation point is at the bottom of the Euphotic Zone
Light Response Curve
Compensation Point
- Light intensity at which gross carbon fixation via photosynthesis is balanced by CO2 -C losses via respiration
- Explains why it is more difficult for leaves and plants to survive and grow in the subcanopy
- At the compensation point, a plant is neither losing or gaining carbon (net)