  We have studied the interstellar extinction in a field of ~3' x 3' at the
core of the 30 Doradus nebula, including the central R136 cluster, in the Large
Magellanic Cloud. Observations at optical and near-infrared wavelengths,
obtained with the WFC3 camera on board the Hubble Space Telescope, show that
the stars belonging to the red giant clump are spread across the
colour-magnitude diagrams because of the considerable and uneven levels of
extinction in this region. Since these stars share very similar physical
properties and are all at the same distance, they allow us to derive the
absolute extinction in a straightforward and reliable way. Thus we have
measured the extinction towards about 180 objects and the extinction law in the
range 0.3 - 1.6 micron. At optical wavelengths, the extinction curve is almost
parallel to that of the diffuse Galactic interstellar medium. Taking the latter
as a template, the value of Rv = 4.5 +/- 0.2 that we measure indicates that in
the optical there is an extra grey component due to a larger fraction of large
grains. At wavelengths longer than ~1 micron, the contribution of this
additional component tapers off as lambda^-1.5, like in the Milky Way,
suggesting that the nature of the grains is otherwise similar to those in our
Galaxy, but with a ~2.2 times higher fraction of large grains. These results
are consistent with the addition of "fresh" large grains by supernova
explosions, as recently revealed by Herschel and ALMA observations of SN 1987A.
