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 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:

You will often read about "pits" on a CD instead of bumps. They appear as pits on the aluminum side, but on the side the laser reads from, they are bumps.

The incredibly small dimensions of the bumps make the spiral track on a CD extremely long. If you could lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and 5 km long

How optical discs are read

A CD is read by focusing a 780 nm wavelength- infrared level, semiconductor laser through the bottom of the polycarbonate layer. The change in height between pits and lands results in a difference in the way the light is reflected. By measuring the intensity change with a photo-diode, the data can be read from the disc. 

Each string of 1s and 0s corresponds to an electrical signal (a voltage). The DAC (digital-to-analog converter) turns the numbers into voltages. The voltages change 44,000 times per second! The amplifier sends the voltages to the speakers where they turn into a series of sounds.