  It was recently argued that the observed PVLAS anomaly can be explained by
chameleon field theories in which large deviations from Newton's law can be
avoided. Here we present the predictions for the dichroism and the
birefringence induced in the vacuum by a magnetic field in these models. We
show that chameleon particles behave very differently from standard axion-like
particles (ALPs). We find that, unlike ALPs, the chameleon particles are
confined within the experimental set-up. As a consequence, the birefringence is
always bigger than the dichroism in PVLAS-type experiments.
