What is the pit on CD?

Pit and land
The depression part on the Optical track is called information pit and the flat part is called land. Pit and land are used to record information.

CD-R disc has been pressed with spiral groove, known as the "pre-groove", instead the pit of optical track. When burning, CD-R disc in a trench in an organic dye laser irradiation, the formation of bubbles, this is the "pit". Bubbles, once formed, will not be able to restore the status quo, therefore, CD-R can only be written once.

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