Data is stored on a disc, particularly optical discs like CDs, DVDs, and Blu-rays, in the form of microscopic data pits and lands.
Understanding Pits and Lands
At the fundamental level, the information written to an optical disc is represented by physical alterations on its surface. As stated in the reference:
Data is stored on the disk in the form of microscopic data pits and lands. The pits are etched into a reflective layer of recording material. The lands are the flat, unindented areas surrounding the pits.
- Pits: These are tiny depressions or indentations created on the disc's surface during manufacturing (for read-only discs like CDs) or recording (for recordable/rewritable discs). They are "etched into a reflective layer."
- Lands: These are the flat, smooth areas on the disc's surface between the pits. They are the "flat, unindented areas surrounding the pits."
These physical features—the presence or absence of a pit compared to a land—are used to represent the binary data (0s and 1s) that computers understand.
How Pits and Lands Represent Data
When a disc is read, a laser beam is directed onto the spinning surface. A light sensor detects the reflection pattern of the laser beam.
- When the laser hits a land, the light reflects directly back to the sensor.
- When the laser transitions from a land to a pit, or from a pit to a land, the change in surface causes the light to scatter or interfere, resulting in less light reflecting back to the sensor.
It is typically the transition between a pit and a land (or vice versa) that represents a '1' bit, while the absence of a transition over a certain distance represents a '0' bit. The length of the lands and pits is standardized to ensure accurate data reading.
Components Involved in Data Storage
The storage mechanism relies on specific layers within the disc structure:
- Reflective Layer: This layer (often made of aluminum, gold, or silver) is crucial for the laser beam to reflect off the disc's surface. Pits are etched into this layer.
- Recording Material: The nature of this material varies depending on the disc type and its intended use (e.g., read-only, recordable, rewritable). As the reference notes, "The type of material selected for the recording material depends on how the disk is used." This layer is where the physical pits are formed.
- Protective Layer: A clear plastic layer coats the reflective and recording layers to protect them from scratches and dust.
In summary, data on optical discs is physically encoded as patterns of pits and lands on a reflective surface, which are then read by a laser detecting changes in light reflection.
