  We report the optical and magneto-optical properties of K2V3O8, an S=1/2
quasi-two-dimensional Heisenberg antiferromagnet. Local spin density
approximation electronic structure calculations are used to assign the observed
excitations and analyze the field dependent features. Two large magneto-optical
effects, centered at ~1.19 and 2.5 eV, are attributed to field-induced changes
in the V 4+ d to d on-site excitations due to modification of the local crystal
field environment of the VO5 square pyramids with applied magnetic field. Taken
together, the evidence for a soft lattice, the presence of vibrational fine
structure on the sharp 1.19 eV magneto-optical feature,and the fact that these
optical excitations are due to transitions from a nearly pure spin polarized V
d state to hybridized states involving both V and O, suggest that the
magneto-dielectric effect in K2V3O8 is driven by strong lattice coupling.
