How CD Writer records data?

Writing and erasing in a CD is carried out by a laser spot focused to about a micrometer in size, which converts the transparent light sensitive material of a `land’ spot to opaque state of the `pit’ spot.

• Please note that the CD lasers with a wavelength of 780 nm were used, being within infrared range. For DVDs, the wavelength was reduced to 650 nm (red color), and the wavelength for Blu-ray Disc was reduced to 405 nm (violet color).
  

The sensitive data region of a CD/DVD is made up of a special material which can be called phase-change material. It can change from one phase (crystalline) to another (amorphous) when it is heated and cooled. The material used is chosen because it reflects light differently in the two states. The amorphous state reflects less light than the crystalline state. So, by starting with a disc surface in the crystalline state, heating with the laser can change small spots to the amorphous state and the rapid cooling of the spot causes the material to freeze in the amorphous state. This appears dark during `reading.’ During the erasure of an amorphous spot, it is converted back to crystalline state by a process known as annealing, accomplished by heating the material to a lower temperature.