Optical storage means that the technology deployed makes use of optical effects and physics to enable storage.
A DVD writer \ reader includes a laser beam to write darker pits onto the surface of the disk - a dark pit represents one type of bit (logic 1 for example) and a non-burned patch represents the other type of bit (logic 0 for example). A laser beam is then reflected off the surface to read the bits.
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Click on this link: Use of laser in DVD