  The scattering properties of the dust originating from debris discs are still
poorly known. The analysis of scattered light is however a powerful
remote-sensing tool to understand the physical properties of dust particles
orbiting other stars. Scattered light is indeed widely used to characterise the
properties of cometary dust in the solar system.
  We aim to measure the morphology and scattering properties of the dust from
the debris ring around HR4796A in polarised optical light. We obtained
high-contrast polarimetric images of HR4796A in the wavelength range 600-900nm
with the SPHERE / ZIMPOL instrument on the Very Large Telescope.
  We measured for the first time the polarised phase function of the dust in a
debris system over a wide range of scattering angles in the optical. We confirm
that it is incompatible with dust particles being compact spheres under the
assumption of the Mie theory, and propose alternative scenarios compatible with
the observations, such as particles with irregular surface roughness or
aggregate particles.
