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Polarized extinction and emission from dust in the interstellar medium (ISM)are hard to interpret, as they have a complex dependence on dust opticalproperties, grain alignment and magnetic field orientation. This isparticularly true in molecular clouds. The data available today are not yetused to their full potential.  The combination of emission and extinction, in particular, providesinformation not available from either of them alone. We combine data from thescientific literature on polarized dust extinction with Planck data onpolarized emission and we use them to constrain the possible variations in dustand environmental conditions inside molecular clouds, and especiallytranslucent lines of sight, taking into account magnetic field orientation.  We focus on the dependence between \lambda_max -- the wavelength of maximumpolarization in extinction -- and other observables such as the extinctionpolarization, the emission polarization and the ratio of the two. We set out toreproduce these correlations using Monte-Carlo simulations where the relevantquantities in a dust model -- grain alignment, size distribution and magneticfield orientation -- vary to mimic the diverse conditions expected insidemolecular clouds.  None of the quantities chosen can explain the observational data on its own:the best results are obtained when all quantities vary significantly across andwithin clouds. However, some of the data -- most notably the stars with lowemission-to-extinction polarization ratio -- are not reproduced by oursimulation. Our results suggest not only that dust evolution is necessary toexplain polarization in molecular clouds, but that a simple change in sizedistribution is not sufficient to explain the data, and point the way forfuture and more sophisticated models.

Interplay of dust alignment, grain growth and magnetic fields in polarization: lessons from the emission-to-extinction ratio