CD
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Optical discs used originally for storing and playback sound tracks but is developed later to store data, rewrite-ability and much more features, and is the basic and popular form of data storage even when memory sticks or external hard drive are developed strongly. We will have a look at 3 most popular type: CD- DVD- Bluray
Physical Detail
A standard disc form of latter development in optical format 
Physical detail:
A standard CD is 1.2mm thick, 15-20grams in weight, diameter of 120mm. It consists of 4 layers as stated from the figure: a polycarbonate layer that contain data, aluminum or gold layer for reflective purpose, film/lacquer layer to provide protection to previous important parts and the most outer layer used for label printing
From the center outward, components are: the center spindle hole (15 mm), the first-transition area (clamping ring), the clamping area (stacking ring), the second-transition area (mirror band), the program (data) area, and the rim
Data is represented as tiny indentations known as "pits", encoded in a spiral track moulded into the top of the polycarbonate layer. The areas between pits are known as "lands"
CD
The elongated bumps that make up the track are each 0.5 µm wide, a minimum of 0.83 µm long and 125 nm high. They look something like this: 
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