Using spectra of normal emission line galaxies from the First Data Release of the Sloan Digital Sky Survey (SDSS) we have investigated the relations between the extinction C(Hb) as derived from the Ha/Hb emission line ratio and various global parameters of the galaxies. Our main findings are that:

1) C(Hb) is linked with the galaxy spectral type and colour, decreasing from early- to late-type spirals.
2) C(Hb) increases with increasing metallicity.
3) C(Hb) is larger in galaxies with an older stellar population.
4) C(Hb) is larger for more luminous galaxies.
5) The extinction of the stellar light is correlated with both the extinction of the nebular light and the intrinsic galaxy colours.

We propose phenomenological interpretations of our empirical results. We have also cross-correlated our sample of SDSS galaxies with the IRAS data base. Due to the lower redshift limit imposed to our sample and to the detection limit of IRAS, such a procedure selected only luminous infrared galaxies. We found that correlations that were shown by other authors to occur between optical and infrared properties of galaxies disappear when restricting to luminous infrared galaxies.

We also found that the optical properties of the luminous infrared galaxies in our SDSS sample are very similar to those of our entire sample of SDSS galaxies. This may be explained by the IRAS luminosity of the galaxies originating in the regions that formed massive stars less than 1 Myr ago, while the opacity of galaxies as derived from the Ha/Hb emission line ratio is due to diffuse dust. We show some implications of our empirical results on the determination of global star formation rates and total stellar masses in normal galaxies.