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 Sense of Direction A Sense of Direction A Sense of Direction is an album led by vibraphonist and composer Walt Dickerson recorded in 1961 and released on the New Jazz label. AllMusic awarded the album 4½ stars with its review by Steve Huey stating, "If it's a shade less challenging and inventive than "This Is Walt Dickerson!", "A Sense of Direction" compensates by making Dickerson's innovations more accessible and inviting". Down Beat reviewer Pete Welding wrote "if it is only slightly less adventurous than his initial recording, it is still an ardent collection -- primarily of ballads -- treated with unflagging invention and