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 Sound Like Someone Trying Not to Make a Sound A Sound Like Someone Trying Not to Make a Sound A Sound Like Someone Trying Not to Make a Sound (Doubleday Books for Young Readers, September 28, 2004, ) is a children's picture book written by John Irving and illustrated by Tatjana Hauptmann. The title was originally used for a children's book written by a fictional character (Ted Cole) in Irving's 1998 novel "A Widow for One Year". The story takes place in a house, late at night. A young child, asleep in his bed, awakes to hear a sound he describes as "a sound like someone trying not to