Radiation Basics

Abstract

The modes of energy transport discussed so far (conduction, convection, and latent heat) all are somewhat intuitive. Radiative energy transport, on the other hand, is not intuitive at all. Radiant energy is transferred by photons, discrete bundles of electromagnetic energy that travel at the speed of light (c = 3 × 10¹⁰ m/s in vacuum) and behave both as particles and waves. These photons are emitted or absorbed by matter as a result of 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:

$$ e = \frac{{hc}} {\lambda } $$

((10.1))

where h is Planck’s constant (6.63 × 10⁻³⁴ J s) and λ is the wavelength of the photon. Thus green photons, having a wavelength of 0.55μm would have an energy

$$ e = \frac{{6.63 \times 10^{ - 34} Js \times 3 \times 10^8 \frac{m} {s}}} {{5.5 \times 10 - ^{ - 7} m}} = 3.6 \times 10^{ - 19} J. $$