  Optical precision experiments are a powerful tool to explore hidden sectors
of a variety of standard-model extensions with potentially tiny couplings to
photons. An important example is given by extensions involving an extra light
U(1) gauge degree of freedom, so-called paraphotons, with gauge-kinetic mixing
with the normal photon. These models naturally give rise to minicharged
particles which can be searched for with optical experiments. In this paper, we
study the effects of paraphotons in such experiments. We describe in detail the
role of a magnetic field for photon-paraphoton oscillations in models with
low-mass minicharged particles. In particular, we find that the upcoming
light-shining-through-walls experiments are sensitive to paraphotons and can
distinguish them from axion-like particles.
