Possibility of a Light Pulse with Speed Greater than c

In two models it is shown that a light pulse propagates from a vacuum into certain media with velocity greater than that of a light in a vacuum (c). By numerical calculation the propagating properties of such a light are given. Recently L. J. Wang and his collaborators in their experiment [1] have found that the group velocity of a laser pulse in particularly prepared atomic caesium gas can much exceed that of light in a vacuum (c). Here we will explore the problem from a theoretical piont of view. Many years ago A. Sommerfeld rigorously proved [2] that the velocity of light pulse can not exceed c in absorption media. We call this conclusion as basic theorem afterwardz. In the following we will first examine why usually the basic theorem holds and in what condition it will be violated. Then two models are proposed, where with precise numerical calculation we will show that the basic theorem is indeed violated. Therefore the properties of light propagating in media of the models are given. We believe that these properties may appears in more realistic models.