Since the birth of quantum mechanics, interference effects between different paths have represented one of the most fascinating signatures of this theory. Systems of three-level atoms show a remarkable example of this effect. If one couples a long-living metastable state to an excited state by means of a strong laser field and one then probes the ground-excited transition of the dressed atom by means of another weak laser field, one will see no absorption and all the radiation will be transmitted through the atomic medium. This phenomenon is called Electromagnetically Induced Transparency (EIT) and it can be explained in terms of quantum interference between different atomic excitation schemes. An atomic medium under EIT conditions shows a polaritonic dispersion, with three branches near the Raman two-photon transition between the ground and metastable states. The central polariton is characterized by a huge reduction of the group velocity of light which can be controlled by the intensity of the dressing field. The energy of the incoming radiation then coherently oscillates between electromagnetic field and atomic excitations: this gives rise to a trapping effect which eventually slows down the radiation, as can be seen in a full quantum treatment. In the limit of a vanishing coupling field, all the radiation energy is stored as a coherent collective atomic excitation. We have first studied the dispersion of light for a lattice of three-level atoms in the linear regime by means of the semi-classical Transfer Matrix technique: we have obtained the different polariton bands and the reflection spectrum at the interface of the atomic medium. The latter shows a dip in correspondence of the Raman resonance which is associated to the refractive index going to 1. We have verified that this feature still holds in the case of a hole (lack of one atom) in the lattice. Instead, in the case of an impurity consisting of an undressed two-level atom, a peak appears at Raman resonance and the radiation is fully scattered back. This case can be important for the behaviour of the system in the non-linear regime. Beyond the steady-state picture, the slow propagation of light opens the possibility of modulating the dressing laser field in order to manipulate in real time a travelling polariton. This is an example of a dynamical photonic structure; similar schemes have been suggested for photon lifter applications. We are presently investigating the effect of dynamical changes of the dressing field on the scattering amplitude from a defect: this problem is of interest in the perspective of probing the new quantum phases of ultracold gases by means of slow light.