One of the most important modes of energy transfer between organisms and their environment is radiation. Radiant energy is transferred by photons, discrete bundles of energy that travel at the speed of light (c = 3 × 108 m/s) and have properties similar to both particles and waves. These photons are emitted or absorbed by matter as a result of discrete quantum jumps in electronic energy levels in atoms or changes in vibrational and rotational energy levels in molecules . The wavelength of the radiation is uniquely related to the photon energy in an equation due to Planck: e = hc/λ, where h is Planck’s constant (6.63 × 10-34 J s) and λ is the wavelength. Thus green photons, having a wavelength of 0.55 µm would have an energy e = 6.63 × 10-34 J s × 3 × 108 m/s ÷ 5.5 × 10-7m = 3.6 × 10-19 J. This number is inconveniently small, and the main purpose of this type of calculation is to find the energy available for photochemical reactions anyway, so the energy contained in a mole (6.02 × 1023) of photons is often calculated. A mole of photons is called an Einstein (E). The energy of photons at 0.55 μm is 6.02 × 1023 × 3.6 × 10-19 = 2.2 × 105 J/E.
KeywordsRadiant Energy Elevation Angle Radiant Emittance Solar Elevation Angle Solar Beam
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