All bodies emit radiation to their surroundings through electromagnetic waves due to the conversion of the internal energy of the body into radiation. Reflectivity, absorptivity, and transmissivity of bodies are defined. Planck’s law for spectral distribution of emissive power of a blackbody is presented in Sect. 10.4, which has been used to determine Stefan–Boltzmann’s equation of total emissive power of a blackbody. Wein’s displacement law is given in the next section. Concept of gray body has been presented. Kirchhoff’s Law has been stated and proved. Equality of emissivity and absorptivity has been established. Lambert’s cosine law has been presented and utilized to determine intensity of radiation in terms of total emissive power of a blackbody.