  In this paper, we report on the optical absorption in porous silicon. We
model the absorption process assuming that porous silicon is a pseudo 1D
material system having a distribution of band gaps. We show that in order to
explain the absorption we specifically need to invoke - (a) k is not conserved
in optical transitions, (b) the oscillator strength of these transitions
depends on the size of the nanostructure in which absorption takes place and
(c) the distribution of band gaps significantly influences the optical
absorption. We also show that it is not possible to extract the band gap of
porous silicon from a plot of $\sqrt{\alpha\hbar\omega}$ vs $\hbar\omega$.
