CDs are made up of three layers: a clear plastic layer about 1.2 mm thick, on top of which a thin, reflective aluminum layer is laid, followed by a thin, protective acrylic layer. Data is recorded on the inner face of the clear plastic layer facing the reflective aluminum layer, in the form of pits that are 0.5 microns in width, and several microns in length. When CD data is read, laser light is focused through the clear plastic layer, and data gathered by reading the differences in reflected light intensities from the aluminum layer caused by the pits.