Apparatus for detection of an edge of a generally disk-shaped workpiece, such as a semiconductor wafer, includes a light source positioned to direct a light beam at the surface of the workpiece near the edge thereof such that a first part of the light beam passes the workpiece and a second part of the light beam is intercepted by the workpiece. An angle between the light beam and a normal to the surface is equal to or greater than a critical angle that produces total internal reflection of the light beam in the workpiece. The apparatus further includes a mechanism for rotating the workpiece and a light sensor positioned to sense the first part of the light beam and to generate an edge signal that represents the edge of the workpiece as the workpiece is rotated. The apparatus may be used for sensing orientation and location of semiconductor wafers of different materials, including those which are transparent to the light beam.