Summary
Highlights
Thermal radiation is the emission of electromagnetic waves by any object with a temperature greater than absolute zero. These waves travel at the speed of light and can occur in a vacuum, unlike conduction and convection. Engineers use this knowledge for various designs like photovoltaic cells and energy-efficient structures.
Electromagnetic waves are characterized by their wavelength. Thermal radiation falls within a wavelength range of 0.1 to 100 microns, overlapping with UV, visible, and infrared parts of the spectrum. The total energy radiated per unit area per second is called emissive power (E), measured in Watts per square meter.
A theoretical 'black body' is a perfect emitter that radiates the maximum possible thermal radiation at a given temperature. Its emissive power is calculated using the Stefan-Boltzmann law, which states that emissive power is proportional to the fourth power of the absolute temperature. Higher temperatures lead to significantly more radiated energy.
The electromagnetic waves emitted by thermal radiation have varying wavelengths. As temperature increases, the distribution of emitted power shifts towards shorter wavelengths. Wien's displacement law describes the relationship between temperature and the wavelength at which most power is emitted, allowing astronomers to estimate star temperatures.
Real objects do not behave as perfect black bodies. Their emissive power is less than that of a black body and is quantified by emissivity (Epsilon), a term that modifies the Stefan-Boltzmann law. Emissivity depends on surface properties and can be a significant design consideration, for example, in choosing coatings for heat tanks or electronics enclosures.
While black bodies are diffuse emitters, radiating evenly in all directions, real surfaces emit radiation unevenly. Analyzing this directional dependence can be complex, but often, assuming objects act as diffuse emitters with an average or normal emissivity is a reasonable simplification.
Irradiation (G) is the total thermal radiation reaching a body per unit area. When radiation strikes a surface, it can be absorbed, reflected, or transmitted. These phenomena are quantified by absorptivity (Alpha), reflectivity (Rho), and transmissivity (Tau), which sum up to one. A black body completely absorbs all incident radiation.
The way radiation is exchanged between surfaces depends on their relative positioning, described by a 'view factor' (F). This geometric parameter represents the fraction of energy radiated from one surface that reaches another. The reciprocity rule relates view factors between two surfaces, enabling calculations of net heat transfer.
While often described as electromagnetic waves, Planck's law, derived by Max Planck in the early 20th century, revealed that radiation is emitted in discrete packets of energy called photons. This groundbreaking discovery, which resolved the 'ultraviolet catastrophe,' laid the foundation for quantum mechanics.