CD-ROM can only be read from and it was written to during manufacturing. A CD-ROM has three layers. A transparent polycarbonate plastic layer that has millions of tiny pumps and flat areas that were produced during manufacturing and that represent bits stored along the disk track. This polycarbonate plastic layer is covered with a reflective aluminum layer which in turn is covered with a thin plastic layer for protection. The drive’s read head projects a low-intensity laser beam onto the polycarbonate plastic layer. The aluminum covering a flat area reflects the beam into a sensor and this is interpreted as a 1. On the other hand, the aluminum covering a bump diffuses the beam away from the sensor and this is interpreted as a 0.
