Focal-plane imaging with microwave and millimeter wave has many potential applications. The bad resolution is a main shortcoming of this technique at present. In this paper, influence of frequency of electromagnetic wave on the refractive energy is analyzed. According to the refractive mechanism of electromagnetic induction, the principle for designing artificial materials with enough refractive ability is proposed. The material of artificial molecules is fabricated with cage-shaped granules of conductor (CGC). The refractive index of CGC material to microwave was measured by assembling CGC prism with PS (polystyrene) prism or SGC (solid granule of conductor) prism. The assemblies greatly eliminate displacement of the outgoing microwave caused by diffraction. This is a key point to achieve direct measurement of the refractive index under the microwave. CGC material shows a considerable refractive ability. SGC material and PS show little refractive ability. The results are useful in promoting image technique of microwave and millimeter-wave by improving the refractive ability and eliminating detriment of the diffraction on the lenses.