Applications of Fresnel diffraction from the edge of a transparent plate in transmission

When a transparent plane-parallel plate is illuminated at the edge region by a quasi-monochromatic parallel beam of light, diffraction fringes appear on a plane perpendicular to the transmitted beam direction. The sharp change in the refractive index at the plate boundary imposes an abrupt change on the phase of the illuminating beam that leads to the Fresnel diffraction. The visibility of the diffraction fringes depends on the plate thickness, refractive index, light wavelength, and angle of incidence. In this report we show that, by recording the visibility repetition versus incident angle, one can measure the plate refractive index, its thickness, and light wavelength very accurately. It is also shown that the technique is indispensable for specifying color dispersion in plate shape samples. The technique is applied to the measurement of dispersion in a fused silica plate and the refractive indices of soda lime slides.