  Extinction in galaxies affects their observed properties. In scenarios
describing the distribution of dust and stars in individual disk galaxies, the
amplitude of the extinction can be modulated by the inclination of the
galaxies. In this work we investigate the inclination dependency in composite
spectra of star-forming disk galaxies from the Sloan Digital Sky Survey Data
Release 5. In a volume-limited sample within a redshift range 0.065-0.075 and a
r-band Petrosian absolute magnitude range -19.5 to -22 which exhibits a flat
distribution of inclination, the inclined relative to face-on extinction in the
stellar continuum is found empirically to increase with inclination in the g,
r, and i bands. Within the central 0.5 intrinsic half-light radius of the
galaxies, the g-band relative extinction in the stellar continuum for the
highly-inclined objects (axis ratio b/a = 0.1) is 1.2 mag, agreeing with
previous studies. The extinction curve of the disk galaxies is given in the
restframe wavelengths 3700-8000 angstrom, identified with major optical
emission and absorption lines in diagnostics. The Balmer decrement remains
constant with inclination, suggesting a different kind of dust configuration
and/or reddening mechanism in the HII region from that in the stellar
continuum. One factor is shown to be the presence of spatially non-uniform
interstellar extinction, presumably caused by clumped dust in the vicinity of
the HII region.
