An apparatus and a process for storing and retrieving information comprising an optical disk having a substrate and a recording layer of a predetermined optical thickness disposed on the substrate. The indices of refraction of the substrate and recording layer are unequal, causing Fresnel reflections from the surface of the layer and the boundary between the substrate and the layer. The optical thickness of the layer is selectively changeable to a plurality of distinct values different from the predetermined value, thereby changing the intensity reflectance of the optical disk to one of a plurality of values. Differences in the intensity of light reflected or transmitted by the disk are detected and serve to represent information according to a multi-level rather than binary digital system.