This happens because the linear, or tangential, speed of the bumps is equal to the radius times the speed at which the disc is revolving. As the laser moves outward from the center of the disc, the bumps move past the laser at an increasing speed. As the DVD is played, the tracking system has to move the laser continually outward. This centering is the job of the tracking system. The hardest part of reading a DVD is keeping the laser beam centered on the data track. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes. The bumps reflect light differently than the "lands," the flat areas of the disc, and the opto-electronic sensor detects that change in reflectivity. The laser beam passes through the polycarbonate layer, bounces off the reflective layer behind it and hits an opto-electronic device, which detects changes in light.
The laser can focus either on the semi-transparent reflective material behind the closest layer, or, in the case of a double-layer disc, through this layer and onto the reflective material behind the inner layer. To work properly, the DVD player must focus the laser on the track of bumps.