  While the deflection of light is achromatic in General Relativity, it is not
always so in several new-physics models (e.g. certain quantum-gravity and
string-inspired models, models with nonminimal photon-gravity coupling or with
massive photon etc.). We discuss how parameters of these models may be
constrained by precise astrometry at different wavelenghts. From published
observations of the gravitational lens MG J2016+112, we obtain world-best
limits on chromatic gravitational deflection of light (and the unique limit on
the photon mass relevant for distance scales >Mpc). We also outline prospects
for further improvement of these limits.
