by an arrow a by means of a voice coil motor 30a to move the light beam radially across the disk 34 the galvano mirror 30 in fact is oscillated by supplying a tracking servo signal to the voice coil motor 30a thus the light beam can follow an information track on the disk 34 the objective lens 32 is moved in directions shown by an arrow b to focus the light beam onto the surface of the disk 34 by means of a voice coil motor 32a driven by a focus servo signal on the optical disk 34 a spiral information track is formed this track is composed of a succession of pits and islands disposed alternately along a spiral path the pit depth is one eighth the wavelength of the light beam from the light source 22 a light beam reflected from the disk 34 is passed through the objective lens 32 and reflected again by the galvano mirror 30 to the beam splitting prism 26 via the quarterwave plate 28 at the beam splitting prism 26 the reflected beam is further reflected and then passed through a lens 38 and a knife edge or a cylindrical lens 40 to a photodetector 42 the photodetector 42 is composed of two or more photoelectric transducers such as photodiodes for detecting.