The Formula for Determining Wavelength by Using a Diffraction Grating with Non-perpendicular Incident Light

We presented a method to determine wave length by measuring the angles of the incident and refracted light in diffraction gratings.This method has an advantage to avoid the problem of the exact adjustments of the mirrors due to the double cross images induced from the double reflections in diffraction grating,and reduces the error in wave length measurements with non-perpendicular incident light.