  The standard model of cosmology is based on the hypothesis that the Universe
is spatially homogeneous and isotropic. When interpreting most observations,
this cosmological principle is applied stricto sensu: the light emitted by
distant sources is assumed to propagate through a Friedmann-Lema\^itre
spacetime. The main goal of the present thesis was to evaluate how reliable
this assumption is, especially when small scales are at stake. After having
reviewed the laws of geometric optics in curved spacetime, and the standard
interpretation of cosmological observables, the dissertation reports a
comprehensive analysis of light propagation in Swiss-cheese models, designed to
capture the clumpy character of the Universe. The resulting impact on the
interpretation of the Hubble diagram is quantified, and shown to be relatively
small, thanks to the cosmological constant. When applied to current supernova
data, the associated corrections tend however to improve the agreement between
the cosmological parameters inferred from the Hubble diagram and from the
cosmic microwave background. This is a hint that the effect of small-scale
structures on light propagation may become non-negligible in the era of
precision cosmology. This motivated the development of a new theoretical
framework, based on stochastic processes, which aims at describing small-scale
gravitational lensing with a better accuracy. Regarding the isotropy side of
the cosmological principle, this dissertation addresses, on the one hand, the
potential effect of a large-scale anisotropy on light propagation, by solving
all the equations of geometric optics in the Bianchi I spacetime. On the other
hand, possible sources of such an anisotropy, namely scalar-vector models for
inflation or dark energy, are analysed. Most of them turn out to be excluded as
physically viable theories.
