British scientist and astronomer William Herschel was investigating the association between heat and light with a thermometer and a prism when he found that the temperature was highest in the region just beyond the red portion of the visible light spectrum. Infrared light, which lies beyond the longer red wavelengths of visible light, was the first "invisible" form of electromagnetic radiation to be discovered. The link between light, electricity, and magnetism was not immediately obvious to early scientists who were experimenting with the fundamental properties of light and matter. Visible light represents only a small portion of the entire spectrum of electromagnetic radiation (as categorized in Figure 1), which extends from high-frequency cosmic and gamma rays through X-rays, ultraviolet light, infrared radiation, and microwaves, down to very low frequency long-wavelength radio waves. The term electromagnetic radiation, coined by Sir James Clerk Maxwell, is derived from the characteristic electric and magnetic properties common to all forms of this wave-like energy, as manifested by the generation of both electrical and magnetic oscillating fields as the waves propagate through space. The mechanisms by which visible light is emitted or absorbed by substances, and how it predictably reacts under varying conditions as it travels through space and the atmosphere, form the basis of the existence of color in our universe. Electromagnetic radiation, the larger family of wave-like phenomena to which visible light belongs (also known as radiant energy), is the primary vehicle transporting energy through the vast reaches of the universe. 20.3 and 20.4, we show how radiation, conduction and convection can influence each other, describing how to design a solar panel.Visible light is a complex phenomenon that is classically explained with a simple model based on propagating rays and wavefronts, a concept first proposed in the late 1600s by Dutch physicist Christiaan Huygens. 20.2, we show how non-idealities are accounted for, defining emissivity and view factor. 20.1 the law of Stefan-Boltzmann is derived, showing that the radiant flux emitted by an ideal object (i.e., a blackbody) depends only on the temperature of its surface. Most energy of this type is in the infra-red region of the electromagnetic spectrum, although some of it is in the visible region, and should not be confused with other forms of electromagnetic radiation, such as radio waves, x-rays, or gamma rays. In this Section, we will consider the form of electromagnetic radiation that is connected to heat transfer, that is thermal radiation. So, for example, gases in a combustion chamber lose more than 90% of their energy by radiation. Accordingly, radiation is the dominant form of energy transport in furnaces, because of their high temperature, and in cryogenic insulation, because of the vacuum existing between particles. Also, radiation is much more dependent on temperature, compared to heat convection and diffusion. Unlike the other modality of energy transport, i.e., convection and diffusion, radiation does not need a medium, such as air or a metal, to propagate and, in fact, it can move across the void, as it happens with the solar energy reaching the earth surface. Radiant heat transfer consists of a transfer of energy through the electromagnetic waves that are emitted by any material object because of its temperature.
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