Radiation Fundamentals
Radiation refers to the emission of electromagnetic waves from the surface of an object. This
radiation has characteristics of both waves and particles, which leads to a description of the radiation
as being composed of photons. The photons generally travel in straight lines from points of emission
to another surface, where they are absorbed, reflected, or transmitted. This electromagnetic radiation
exists over a large range of wavelengths that includes x-rays, ultraviolet radiation, visible light, and
infrared or thermal radiation, as shown in Figure 8.26. The thermal radiation emitted from an object
is related to its temperature and has wavelengths ranging from approximately 10
7 to 10
3 m. It is
necessary to understand two key aspects of radiative heat transfer in relation to temperature
measurements. First, the radiation emitted by an object is proportional to the fourth power of its
temperature. In the ideal case, this may be expressed as