We present new multicolor photopolarimetry of stars behind the Southern Coalsack. Analyzed together with multiband polarization data from the literature, probing the Chamaeleon I, Musca, rho Opiuchus, R CrA, and Taurus clouds, we show that the wavelength of maximum polarization ({lambda}_max_) is linearly correlated with the radiation environment of the grains. Using far-infrared emission data, we show that the large scatter seen in previous studies of {lambda}_max_ as a function of Av is primarily due to line-of-sight effects causing some Av measurements to not be a good tracer of the extinction (radiation field strength) seen by the grains being probed.