  The empirical reddening function for starburst galaxies generated by Calzetti
and her co-workers has proven very successful, and is now used widely in the
observational literature. Despite its success, however, the physical basis for
this extinction law, or more correctly, attenuation law remains weak. Here we
provide a physical explanation for the Calzetti Law based on a turbulent
interstellar medium. In essence, this provides a log-normal distribution of
column densities, giving a wide range of column densities in the dusty
foreground screen. Therefore, extended sources such as starburst regions or HII
regions seen through it suffer a point-to-point stochastic extinction and
reddening. Regions of high column densities are "black" in the UV, but
translucent in the IR, which leads to a flatter extinction law, and a larger
value of the total to selective extinction, R_V. We fit the Calzetti Law, and
infer that the variance sigma of the log-normal distribution lies in the range
0.6<sigma<2.2. The absolute to selective extinction R_V is found to be in the
range 4.3 to 5.2 consistent with R_V=4.05+/-0.80 of the Calzetti Law.

Astrophysics