Kirchhoff's law of thermal radiation rarefied material such as air, or transparent material, through which observations can be made. The interface is not a material body and can neither emit nor absorb. It is a mathematical surface belonging jointly to the two media that touch it. It is the site of refraction of radiation that penetrates it and of reflection of radiation that does not. As such it obeys the Helmholtz reciprocity principle. The opaque body is considered to have a material interior that absorbs all and scatters or transmits none of the radiation that reaches it through refraction at the interface. In this sense
A Boy, a Girl and a Bike A Boy, a Girl and a Bike A Boy, a Girl and a Bike is a 1949 British romantic comedy film directed by Ralph Smart and starring John McCallum, Honor Blackman and Patrick Holt. The film's art direction was by George Provis. The film concerns the romantic escapades and adventures of a Yorkshire cycling club. Young couple Sue (Honor Blackman) and Sam (Patrick Holt) are members of a Yorkshire cycling club, the ‘Wakeford Wheelers’. Romantic complications ensue when wealthy David (John McCallum) becomes smitten with Sue, and joins the club to pursue her, much to Sam's dismay. The "Radio Times"