Abstract: The reason of the emergence of unclear interference fringe in measurement of refractive index of plate glass using spectrometer is analyzed. According to the similar intensity distribution of simulation and observa- tion, the wedge angle of plate glass is the main cause. Based on the contrast of interference stripe varied by reflection of plate glass in different angles, to make sure the students could observe clear interference stripe, reflection of plate glass should be chosen between 0.5 and 0.7.

Key words: spectrometer, multi-beam interference, wedge angle, contrast