  Previous numerical simulations of double-peaked SNe IIb light curves have
demonstrated that the radius and mass of the hydrogen-rich envelope of the
progenitor star can significantly influence the brightness and timescale of the
early-time light curve around the first peak. In this study, we investigate how
Thomson scattering and chemical mixing in the SN ejecta affect the optical
light curves during the early stages of the SNe IIb using radiation
hydrodynamics simulations. By comparing the results from two different
numerical codes (i.e., \stella{} and \snec{}), we find that the optical
brightness of the first peak can be reduced by more than a factor of 3 due to
the effect of Thomson scattering that causes the thermalization depth to be
located below the Rosseland-mean photosphere, compared to the corresponding
case where this effect is ignored. We also observe a short-lived plateau-like
feature lasting for a few days in the early-time optical light curves of our
models, in contrast to typical observed SNe IIb that show a quasi-linear
decrease in optical magnitudes after the first peak. A significant degree of
chemical mixing between the hydrogen-rich envelope and the helium core in SN
ejecta is required to reconcile this discrepancy between the model prediction
and observation. Meanwhile, to properly reproduce the first peak, a significant
mixing of \nifs{} into the hydrogen-rich outermost layers should be restricted.
Our findings indicate that inferring the SN IIb progenitor structure from a
simplified approach that ignores these two factors may introduce substantial
uncertainty.
