Spectroscopic observations of H \alpha and H \beta emission lines of 128 star-forming galaxies in the redshift range 0.75 \leq z \leq 1.5 are presented .
These data were taken with slitless spectroscopy using the G102 and G141 grisms of the Wide-Field-Camera 3 ( WFC3 ) on board the Hubble Space Telescope as part of the WFC3 Infrared Spectroscopic Parallel ( WISP ) survey .
Interstellar dust extinction is measured from stacked spectra that cover the Balmer decrement ( H \alpha /H \beta ) .
We present dust extinction as a function of H \alpha luminosity ( down to 3 \times 10 ^ { 41 } erg s ^ { -1 } ) , galaxy stellar mass ( reaching 4 \times 10 ^ { 8 } M _ { \odot } ) , and rest-frame H \alpha equivalent width .
The faintest galaxies are two times fainter in H \alpha luminosity than galaxies previously studied at z \sim 1.5 .
An evolution is observed where galaxies of the same H \alpha luminosity have lower extinction at higher redshifts , whereas no evolution is found within our error bars with stellar mass .
The lower H \alpha luminosity galaxies in our sample are found to be consistent with no dust extinction .
We find an anti-correlation of the [ O iii ] \lambda 5007 /H \alpha flux ratio as a function of luminosity where galaxies with L _ { H \alpha } < 5 \times 10 ^ { 41 } erg s ^ { -1 } are brighter in [ O iii ] \lambda 5007 than H \alpha .
This trend is evident even after extinction correction , suggesting that the increased [ O iii ] \lambda 5007 /H \alpha ratio in low luminosity galaxies is likely due to lower metallicity and/or higher ionization parameters .