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.