# Radiative Processes in Astrophysics

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## The Black Body

A blackbody is the simplest source: it absorbs and reemits radiation with
100% efficiency. The frequency content of blackbody radiation is given by
the *Planck Function*:

### Derivation

The # density of photons having frequency between and has to equal the # density of phase-space cells in that region, multiplied by the occupation # per cell. Thus:

However,

so we have it. In the limit that :

If :

Note that this tail peaks at . Also,