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
The Sound of a Voice The Sound of a Voice The Sound of a Voice is a 1983 play by American playwright David Henry Hwang. Hwang's fifth play, it is an original ghost story inspired by Japanese folk stories, films, and Noh theater. The play was first produced as part of the production "Sound and Beauty" on November 6, 1983 Off-Broadway at the Joseph Papp Public Theater. It was directed by and featured John Lone. Composer Philip Glass and Hwang adapted it for the opera "The Sound of a Voice" in 2003. That same year, Susan Hoffman directed a film adaptation entitled "Sound of a