Physics · Ch 10 — Thermal Properties of Matter

Heat Transfer: Radiation, Absorptive and Emissive Power

10.11

Radiation is the third and last mode of heat transfer, and it is fundamentally different from both

conduction and convection in one crucial respect: it needs no material medium at all. Heat carried by

radiation travels as electromagnetic waves, which can propagate through completely empty space (a

vacuum) just as readily as through air -- indeed, more readily, since a medium can absorb or scatter some

of the radiation passing through it. This is precisely how the Sun's heat reaches the Earth across the

vast, almost entirely empty vacuum of interplanetary space, where neither conduction nor convection could

possibly operate.

Every body, purely by virtue of being at a temperature above absolute zero, is continuously emitting

thermal radiation from its surface (as a result of the accelerating charges within its constituent atoms

and molecules), and every body is simultaneously absorbing at least some of the radiant energy falling on

it from its surroundings. A body's temperature, over time, evolves according to the net balance between

how much radiant energy it is emitting outward and how much it is absorbing from its surroundings -- if

it emits more than it absorbs, it cools; if it absorbs more than it emits, it warms.

The absorptive power (or absorptivity), usually denoted $a$ , of a surface is defined as the fraction

of the total radiant energy incident on that surface which the surface actually absorbs (the remainder

being either reflected away or, for a partly transparent material, transmitted through). Since it is a

fraction of the incident energy, $a$ is dimensionless and lies between $0$ (a perfect reflector/perfect

transmitter, absorbing nothing) and $1$ . A surface with $a = 1$ -- one that absorbs the whole of the

radiation falling on it, at every wavelength, with no reflection and no transmission at all -- is called a

perfectly black body, an idealisation approached very closely by, for example, a surface coated with

lamp-black (soot).

The emissive power (or emissivity), usually denoted $e$ , of a surface at a given temperature is a

measure of how effectively that surface radiates thermal energy, expressed relative to how much an ideal

black body at the very same temperature would radiate. Like $a$ , $e$ is dimensionless and lies between $0$

and $1$ ; a perfectly black surface, being also the most efficient possible radiator at every wavelength

(a consequence of Kirchhoff's law, taken up in the next section), has $e = 1$ , the maximum value any real

surface can approach but never quite reach.

Dark, rough, matte surfaces (a blackened kettle, soot-coated metal, or matte black paint) tend to have

both a high absorptive power and a high emissive power together, so they absorb incident heat readily and …