Abstract: The interference pattern of Fresnels biprism is recorded by a CMOS camera and converted to intensity distribution curve. It is found that the envelope of the interference fringes fluctuates regularly in the central region and decays rapidly on both sides, two features cannot be observed in the classic double-slit interference pattern. To explain this phenomenon, a simple model is proposed that take the straight-edge diffraction at the ridge of the biprism into consideration. By MATLAB simulation, light intensity distribution matching the observed characteristics is obtained. The model also implies that the spacing between interference fringes gradually narrows in the diffraction regions on both sides. Moreover, the relationship between the number and visibility of interference fringes in the central region with the slit width and the position of the biprism is studied. All the results are in good agreement with the experiment measurements.

Key words: biprism, interference, direct-edge diffraction, visibility