  1-loop quantum corrections are shown to induce large effects on the
refraction index $n$ inside a graphene strip in the presence of an external
magnetic field $B$ orthogonal to it. To this purpose, we use the tools of
Quantum Field Theory to calculate the photon propagator at 1-loop inside
graphene in position space, which leads to an effective vacuum polarization in
a brane-like theory of photons interacting with massless electrons at locations
confined inside the thin strip (its longitudinal spread is considered to be
infinite). The effects factorize into quantum ones, controlled by the value of
$B$ and that of the electromagnetic coupling $\alpha$, and a "transmittance
function" $U$ in which the geometry of the sample and the resulting confinement
of electrons play the major roles. We consider photons inside the visible
spectrum and magnetic fields in the range 1-20\; Teslas. At $B=0$, quantum
effects depend very weakly on $\alpha$ and $n$ is essentially controlled by
$U$; we recover, then, an opacity for visible light of the same order of
magnitude $\pi \alpha_{vac}$ as measured experimentally.
