We show that an Electromagnetically Induced Transparency spectrum is directly related to the density of states of an appropriate Hamiltonian. In our vapor cell ⁸⁷Rb atoms are diffusing in the presence of N₂ buffer gas. We explained how the overlap of the diffusion modes with the EIT beams and the corresponding eigen energies determines the EIT spectrum, we also discussed the dependence of the EIT line shape on the vapor cell shape. We have derived the EIT line shape from the equation (complex diffusion equation), which can describes the evolution of density matrix in time with trap (infinite circular potential well) in presence of buffer gas. Our measured EIT line shape gives a good argument with theoretically predicted line shape.