A phase step diffractometer is disclosed that utilizes Fresnel diffraction from a 1D step. The main part of the device is a step with two flat parallel mirrors on either side. The phase difference (PD) is changed by varying the light incident angle and the step height. The diffracted lights from the step are caught by a CCD connected to a PC. By varying PD, the visibility of the three central diffraction fringes changes. This permits low uncertainties in the measurements of wavelength, coherence length, coherence width, plate thickness, surface topography and fine displacement of objects. In addition, the device can be used in determination of broad spectral line shapes and optical constants of materials.