Lecture 4: Light

Question 1

Planck’s Equation

Answer 1

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.

Question 2

Variation in Light Intensity, Duration, and Directionality (Slide)

Answer 2

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

Question 3

Lambert’s Cosine Law

Answer 3

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

Question 4

Lambert’s Cosine Law

Answer 4

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

Question 5

Lambert’s Cosine Law Equation

Answer 5

Question 6

Lambert’s Cosine Law Diagram

Answer 6

Question 7

Light Intensity Varies With…

Answer 7

Latitude

Season

Time of Day

Aspect

Slope

Question 8

Albedo

Answer 8

Percent of incident solar radiation that is reflected from a surface

Question 9

Leaf Area Index

Answer 9

Total leaf area per unit of ground area

Question 10

Leaf Area Index (LAI) Equation

Answer 10

LAI= Total Lead Area/ projected ground area

Question 11

Beer’s Law

Answer 11

Describes the relationship between leaf area index (L) and available light

Question 12

Beer’s Law Equation

Answer 12

I=I₀e^-kL_i

I= Available light at any height

e= base of natural logarithm (2.718)

L_i= 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

Question 13

Euphotic Zone

Answer 13

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

Question 14

Light Response Curve

Answer 14

Question 15

Compensation Point

Answer 15

• 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)

Question 16

Saturation Point

Answer 16

• Saturation point = light intensity beyond which there is no increase in net C fixation
• At some point, plants are “saturated” with respect to light intensity
  
  • Because of inherent limitations of biochemistry
  • CO₂ may become limiting (diffusion of CO₂ through stomates can’t keep up with photosynthesis
  • Stomates may close if water stress accompanies high light intensity

Question 17

Another Light Response Curve

Answer 17