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菲涅耳双棱镜,是一种重要的分波前双光束干涉装置。因双棱镜的棱角α很小,光源S发出的波列经它折射后,被分割为两束光,它们在交迭区域里迭加而出现等距离的干涉条纹。S_1与S_2为S对双棱镜所成的虚象,幕上的干涉条纹就如同由相干的虚象光源S_1和S_2发出的光束产生的一样。S_1与S_2到屏幕上某点的光程差б=d(y/D),条纹的间隔△y=（D/d）λ。其中D为S_1（或S_2）到幕的距离,λ为入射光的波长,d为两虚光源的距离。（如图一）
Fresnel prism, is an important sub-wavefront two-beam interference device. Due to the small angle α of the biprism, the wave train emitted by the light source S is refracted by it and is split into two beams of light, which are superposed in the overlapping region to form an equidistant interference fringe. S_1 and S_2 are the virtual images made by S versus the biprism, and the interference fringes on the screen are the same as those generated by the beams emitted by coherent virtual light sources S_1 and S_2. The optical path difference between S_1 and S_2 to the point on the screen is б = d (y / D), and the interval of the stripes Δy = (D / d) λ. Where D is the distance between S_1 (or S_2) to the curtain, λ is the wavelength of incident light and d is the distance between two virtual light sources. (Figure 1)