Linear media are predicted to exist whose relative permiability is an operator in the space of quantum states of light. Such media are characterized by a photon statistics--dependent refractive index. This indicates a new type of optical dispersion -- the photon statistics dispersion. Interaction of quantum light with such media modifies the photon number distribution and, in particular, the degree of coherence of light. An excitonic composite -- a collection of noninteracting quantum dots -- is considered as a realization of the medium with the photon statistics dispersion. Expressions are derived for generalized plane waves in an excitonic composite and input--output relations for a planar layer of the material. Transformation rules for different photon initial states are analyzed. Utilization of the photon statistics dispersion in potential quantum--optical devices is discussed.