Effect of negative index of refraction in the propagation of electromagnetic waves

Consider a material slab situated between two half free spaces. An electromagnetic waves is obliquely incident on it. We investigate the effect of index of refraction of the material in the transmission of electromagnetic waves. We study three cases of the material: real positive index of refraction, real negative index of refraction and the refractive index is a function of frequency of the incident waves. We show that the transmission of electromagnetic waves through the slab is indifferent whether the index of refraction is real positive or negative. We also show that the transmission through the slab is completely different if the index of refraction is a function of frequency.