  Super- and subradiance are usually described in the framework of Dicke
collective states, which is an ``atomic picture'' in which the electromagnetic
field only provides an effective interaction between the atoms. Here, we
discuss a complementary picture, in which we describe the propagation and
scattering of light in the atomic medium, which provides a complex
susceptibility and scatterers. This ``optical picture'' is valid in the
linear-optics regime for disordered samples and is mainly relevant at low
density, when the susceptibility and scattering cross-section can be computed
from simple textbook formulas. In this picture, superradiance is a dispersion
effect due to a single scattering event dressed by an effective refractive
index, whereas subradiance is due to multiple scattering. We present numerical
and experimental data supporting our interpretation.
