This chapter briefly summarizes some of the formulas and theorems associated with blackbody
radiation. A small point of style is that when the word "blackbody" is used as an adjective, it is
usually written as a single unhyphenated word, as in "blackbody radiation"; whereas when
"body" is used as a noun and "black" as an adjective, two separate words are used. Thus a black
body emits blackbody radiation. The Sun radiates energy only very approximately like a black
body. The radiation from the Sun is only very approximately blackbody radiation.
2.2 Absorptance, and the Definition of a Black Body.
If a body is irradiated with radiation of wavelength λ, and a fraction a(λ) of that radiation is
absorbed, the remainder being either reflected or transmitted, a(λ) is called the absorptance at
wavelength λ . Note that λ is written in parentheses, to mean "at wavelength λ", not as a
subscript, which would mean "per unit wavelength interval". The fractions of the radiation
reflected and transmitted are, respectively, the reflectance and the transmittance. The sum of the
absorptance, reflectance and transmittance is unity, unless you can think of anything else that
might happen to the radiation.
A body for which a(λ) = 1 for all wavelengths is a black body.
A body for which a has the same value for all wavelengths, but less than unity, is a grey body.
(Caution: We may meet the word "absorbance" later. It is not the same as absorptance.)
2.3 Radiation within a cavity enclosure.
Consider two cavities at the same temperature. We'll suppose that the two cavities can be
connected by a "door" that can be opened or closed to allow or to deny the passage of radiation
between the cavities. We'll suppose that the walls of one cavity are bright and shiny with an
absorptance close to zero, and the walls of the other cavity are dull and black with an
absorptance close to unity. We'll also suppose that, because of the