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. How We Measure the Radiation of Heat This is one of the fundamental relationships of modern physics: _Q__body = ε_A_σ_T_⁴. It was discovered experimentally by Josef Stefan in 1879, and verified theoretically by Ludwig Boltzmann; it is known as the _Stefan-Boltzmann Law_. This formula tells us the amount of radiant energy, Q_body, that will be emitted by a body having the surface area _A_ when it is at the temperature _T_. Temperature, here, is measured in degrees Rankine (°R), or Fahrenheit temperature above absolute zero (to calculate degrees Rankine, add 460 to the temperature in degrees Fahrenheit). The expression ε_A_ is used to show that only a certain fraction of the energy that would leave a perfect black body of area _A_ will actually leave a real body of the same size; the size of this fraction is determined by the body’s emissivity. The symbol σ is a quantity we call the _Stefan-Boltzmann constant_.