When a transparent plate is illuminated perpendicular to its surface by a monochromatic convergent beam of light, the Fresnel fringes, caused by abrupt change in refractive index at the plate boundary, are formed periodical and symmetrical around the optical axis on the observation plane. When the plate is rotating around the direction normal to optical axis and plate boundary, the fringes are moved and formed around the ray that is perpendicular to the plate surface. It is shown theoretically and experimentally that the focal length of lenses can be determined by measurement of the displacement of the central Fresnel fringe.

Keywords: Focal length, Fresnel diffraction, Phase step

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