(epsilon) emissivity, a dimensionless (unitless) measure of a material's effective ability to emit or absorb thermal radiation from its surface ranges from 0 (none) to 1 (maximal) Net heat flow rate emitted (+) or absorbed (−) It is now derived mathematically from Planck's law. It was empirically derived by the Austrian physicist Joseph Stefan in 1879 and theoretically derived by the Austrian physicist Ludwig Boltzmann in 1884. The Stefan-Boltzmann law relates the heat flow rate emitted or absorbed from an object to its temperature (and surface area and darkness). Dark objects lose and gain heat faster than light objects. Hot objects are "brighter" than cold objects. As all forms of electromagnetic radiation transfer internal energy, they could all be called "heat waves". Recall that heat is the transfer of internal energy from one region to another.
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Sometimes this kind of radiation is called "heat waves" but this is a misnomer. Because of this position, this form of radiation is called infrared ( infra being the Latin word for below or within). One of the first to recognize that heat radiation is related to light was the English astronomer William Herschel (1738–1822), who noticed in 1800 that if a thermometer was moved from one end of a prism produced spectrum to the other, the highest temperatures would register below the red band, where no light was visible.
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For most bodies on the Earth, this radiation lies in the infrared region of the electromagnetic spectrum. Heat radiation (as opposed to particle radiation) is the transfer of internal energy in the form of electromagnetic waves.