. Compact Disk is an optical storage medium used to record, store and play audio, video in digital form.
   . It has capacity of storing 650-700 MB of data.
   . Before CD's, cassettes were used to store data.
   . Nowadays, DVDs and Blue Ray discs are replacing CD's as they have more storage capacity.
   . CDs are widely used for music albums as they are compatible with DVD players. 

   What it contains internally?

       . CD is usually 12 cm in diameter which contains 4 layers attached one on the top of another.

       . Polycarbonate layer: Data is encoded by using tiny bumps on the surface.

       . Reflective layer: It is made up of Aluminium which is used to reflect the laser beam back.

       . Protective layer: It is plastic layer which acts as protective to the layers below it.

       . Printing layer: Artwork or labels are printed on this layer using screen printing.

       . With the help of millions of 0's and 1's, CD store data.
       . Pit is considered as 0 and land is considered as 1.
       . These pits and lands are stored in the spiral form starting from the center towards edge of the disc.

   How CDs work?

      . Inside the CD player, when the CD is inserted, it rotates with 500 rpm.
      . Now Laser and Photocell converts this pits and lands into binary digits of 0's and 1's respectively.
      . The circuit converts these digital data into electric signals and send them to speakers.
      . Speakers convert these electric signals into sound and thereby we can hear the sound which is played in CD Player.