狭缝(实光源)和虚光源不在同一平面上，是利用菲涅尔双棱镜干涉测光波波长中经常被忽略的问题，阐明了这一问题引起的测量误差的原理，并推导出了相关误差值的计算过程.分析结果显示，以狭缝平面代替虚光源平面引起的相对误差小于0.01%，远低于实验过程中的其它系统误差.因此，在实验中，通常将狭缝平面位置近似视为虚光源平面的位置，以此来测量与观察屏的距离，忽略虚实光源不共面所引起的测量误差.

The slit (real light source) and the virtual light source are not on the same plane, which is a commonly overlooked problem in measuring the wavelength of light using Fresnel prism interferometry. The principle of measurement error caused by this problem is explained and the process for calculating the associated error values is derived. The analysis indicate that the relative error caused by replacing the virtual light source plane with a slit plane is less than 0.01%, which is significantly smaller than other systematic errors in this experiment. Therefore, in experiments, it is common to approximate the position of the slit plane as the position of the virtual light source plane to measure the distance to the observation screen, neglecting the measurement errors caused by the non-coplanarity of the virtual and real light sources.