Compact disc (CD) and digital video disc (DVD) have equal sizes, but they differ in their storage capacity. Explain.

A Compact Disc (CD) is a type of optical disc used to store digital data. It could be used to store computer files, audio files, MPEG videos etc. A CD usually holds a maximum of 74-80 minutes of sound or 650-700 megabytes of data. A Digital Versatile Disc or a Digital Video Disc (DVD) on the other hand is an optical disc capable of storing up to 4.7 GB of data, more than six times of what a CD can hold. DVDs are often used to store movies at better quality. DVDs can also have interactive menus and bonus features such as deleted scenes and commentaries. It can have one or two sides, and one or two layers of data per side; the number of sides and layers determines how much it can hold. The data on CDs as well as DVDs is recorded in a spiral that starts at the center of the disc. CDs are made from a single layer of polycarbonate with an aluminum recording layer on the top of the disc while DVDs are made from two thin layers of polycarbonate with an aluminum recording layer between the polycarbonate layers. DVDs hold more data because they have a longer spiral to record data.