The physical property of a material that controls the way it absorbs
radiant energy is called its _absorptivity_, and the property that
controls its emission of energy is its _emissivity_. For absorptivity we
use the symbol α; for emissivity we use the symbol ε.
When radiant energy reaches a surface, only a certain part of it is
absorbed; the rest is either reflected, just as light rays are
reflected, or else passes right through it. The absorptivity, α, of a
substance tells us what percentage of radiant energy it will absorb. A
perfect absorber, or _black body_, would absorb all the radiant energy
that reached it. If such an ideal substance existed (which it doesn’t)
we would say it had an α of 1. The actual absorptivities of real
substances are indicated by numbers between 0 and 1: The α of black
velvet cloth, for example, is about 0.97; that of a polished silver
mirror is about 0.08 for solar radiation (absorptivity for most polished
metals for room temperature radiation is even lower).
We measure emissivity, ε, in very much the same way. A hypothetical
black body would emit all the energy it possibly could and have an ε of
1; the emissivities of real substances are indicated by numbers between
0 and 1. For any given frequency (or color) of light, a substance’s
absorptivity and emissivity are equal; however, the total spectrum of
frequencies of the energy absorbed is usually different from that of the
energy emitted.
The ratio between emissivity and absorptivity, α/ε, is very important,
as we shall see later. If this ratio is greater than 1, it means that a
substance absorbs heat faster than it emits it, and thus tends to become
warmer. If the ratio is less than 1, the reverse is true—the surface
emits radiant energy at a faster rate than it absorbs it, and tends to
become cooler.