There is continuous interchange of energy between bodies by the emission and absorption of radiation.

**HEAT TRANSFER BY RADIATION**

There
is continuous interchange of energy between bodies by the emission and
absorption of radiation. If two adjacent surfaces are at different temperatures,
the hotter surface radiates more energy than it receives, and its temperature
falls. The cooler surface receives more energy than it emits, and its
temperature rises. Ultimately, thermal equilibrium is reached. Interchange of
energy con-tinues, but gains and losses are equal.

Of
the radiation that falls on a body, a fraction, *a*, is absorbed, a fraction, *r*,
is reflected, and a fraction, *t*, is
transmitted. These fractions are called the absorptivity, the reflectivity, and
the transmissivity, respectively. Most indus-trial solids are opaque so that
the transmissivity is zero and

*a* + r = 1 (3:15)

*Reflectivity* and, therefore,
absorptivity, depend greatly on the nature of the surface. The limiting case,
that of a body that absorbs all and reflects none of the incident radiation, is
called a blackbody.

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