  We present broad-band photometry of five transits in the planetary system
WASP-36, totaling 17 high-precision light curves. Four of the transits were
simultaneously observed in four passbands (g, r, i, z), using the
telescope-defocussing technique, and achieving scatters of less than 1 mmag per
observation. We used these data to improve the measured orbital and physical
properties of the system, and obtain an optical transmission spectrum of the
planet. We measured a decreasing radius from bluer to redder passbands with a
confidence level of more than 5 sigma. The radius variation is roughly 11
pressure scale heights between the g and the z bands. This is too strong to be
Rayleigh scattering in the planetary atmosphere, and implies the presence of a
species which absorbs strongly at bluer wavelengths.
