Modern Physics for Engineers Lecture slides 23a – Black body radiation and Kirchhoff’s law

This lecture part shows how to envisage a perfect black body for thought experiments and derivations, and goes on to derive Kirchhoff's radiation law, which states that the "absorptivity" of an object (the fraction of incident light it absorbs) must equal its emissivity (the amount of light it emits at a given temperature as a fraction of the light a perfect black body would emit). (slides)