Compact disks provide high quality, cheap data storage with a large capacity. Conventional compact disks can be read many times, but the data cannot be altered. A CD can typically store 74 minutes of music or 650MB of data.
The compact disk is constructed from transparent polycarbonate plastic. The plastic disk (diameter 120mm, thickness 1.2mm) is impressed with tiny bumps. A thin layer of aluminium is deposited on the polycarbonate. The aluminium layer is protected by a layer of acrylic polymer. A decorative paper or paint layer can be added, or sandwiched between layers of acrylic. The compact disk is read by a laser, which is reflected by the bump on the aluminium layer. Each small or large bump represents a 0 or 1 in the binary digital data.
The image above shows a typical compact disk. The scanning electron microscope first shows the surface of the polycarbonate plastic, which gives no evidence of the data storage. Dissolving the acrylic and polycarbonate with solvents reveals the impressed aluminium layer, which can be seen in the last few images at high magnification.