We investigate the interaction between a two-level atomic system and a phase controllable light field. The quantum diffraction of the population probability for the excited state is described analytically. The physical sense of the wave function evolution is analyzed in detail. The polarization of the atoms is shown with the help of Cornu spiral of the wave function and the temporal evolution of the real and imaginary part for the wave function. It is shown that the polarization of the atoms and the temporal evolution of the wave function as well as the propagation of light can be controlled by manipulating the phase of the light field.