We derive a simple approximate model describing the early, up to a few days, 
UV/optical supernova emission, which is produced by the expansion of the 
outer hundredth solar mass of the shock-heated envelope, following the shock 
breakout and preceding the optical emission driven by radioactive decay. Our 
model includes an approximate description of the time dependence of the 
opacity (due mainly to recombination), and of the deviation of the emitted 
spectrum from a black body spectrum. For He envelopes, neglecting the effect 
of recombination may lead to an underestimate of the luminosity by more than 
an order of magnitude. We also show that the relative extinction at different 
wavelengths may be inferred from the light-curves at these wave-lengths, 
removing the uncertainty in the estimate of progenitor radius due to reddening 
(but not the uncertainty in E/M due to uncertainty in absolute extinction). For 
core collapse SN, the characteristics of the early UV/O emission constrain the 
radius of the progenitor star, its envelope composition, and the ratio of the 
ejecta energy to its mass, E/M. For SN Ia, the characteristics of the emission 
may allow one to distinguish between a pure deflagration explosion and a 
delayed detonation (DDT) explosion, and to constrain the detonation and 
deflagration velocities for DDT explosion. The early UV/O observations of the 
type Ib SN2008D and of the type IIp SNLS-04D2dc are consistent with our 
model predictions. For SN2008D we find progenitor radius to be approximately 
10^11 cm, and an indication that the He envelope contains a significant C/O 
fraction.