← IGCSE Computer Science Paper 1

Optical storage: pits and lands

Dents (pits) and flat areas (lands) along a disc's spiral track. A laser reads them by how light reflects: lands reflect it back, pits scatter it. The changes store binary data.

Part of: IGCSE Computer Science 0478 — 3.3 Data Storage — Module 12

Understanding Optical storage: pits and lands in IGCSE Computer Science Paper 1

What it does: Optical discs use a single spiral track of pits and lands, unlike the separate circular tracks of a magnetic hard disk. The difference in reflected light between lands and pits is detected by the sensor and interpreted as binary data, which is how a disc is read. A pit is a dent. A land is the flat area between the pits. A pit scatters the laser light, so the sensor receives much less of it. The sensor detects this change.

Common mistakes: A land is a flat area, so it reflects the laser light straight back to the sensor. Pits are the parts that scatter light. The line should be: light reflects back to sensor. A pit is a dent, so it scatters the laser light instead of reflecting it back. Only lands reflect the light cleanly. The line should be: a pit scatters the light.

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This module's key terms are also in a printable crossword (Modules 304–313) and word search (Modules 304–324).

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Learning
Duo
How is data stored on an optical disc?
As pits and lands along a trackAs tiny magnetised areas on a platter
Matches
A pit is a flat area that reflects the laser back to the sensor
↓
Pits and lands are mixed up. A pit is a dent that scatters the light, while a land is the flat area that reflects it back.
Applying
Meanings
Term
pits and lands
Meaning
Dents (pits) and flat areas (lands) along a disc's spiral track. A laser reads them by how light reflects: lands reflect it back, pits scatter it. The changes store binary data.
Practice
FOR position ← 1 TO 4
Testing
Gaps
light ←
Check Answer
Spots
3light hits a flat land
4light scatters away

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Quick Questions

1. How is data stored on an optical disc?
A. As pits and lands along a track ✓
B. As tiny magnetised areas on a platter
Optical discs store data as pits (dents) and lands (flat areas). Magnetised areas are used in magnetic storage such as HDDs.
2. Which describes a pit on an optical disc?
A. A flat area of the disc surface
B. A tiny dent in the disc surface ✓
A pit is a dent. A land is the flat area between the pits.
3. What happens when the laser shines on a land?
A. The light is reflected back to the sensor ✓
B. The light is scattered away from the sensor
Lands are flat, so they reflect the light straight back to the sensor. Pits scatter the light.
4. What happens when the laser shines on a pit?
A. The light is scattered, so little returns to the sensor ✓
B. All of the light is reflected back to the sensor
A pit scatters the laser light, so the sensor receives much less of it. The sensor detects this change.
5. What is the layout of the track on an optical disc?
A. Many separate circular tracks that never join
B. One long spiral track while the disc spins ✓
An optical disc has a single spiral track that runs from the centre to the edge (or edge to centre) while the disc spins. Separate concentric tracks are a feature of magnetic disks.

Match the Pairs

pit → A tiny dent in the surface of an optical disc that scatters the laser light instead of reflecting it straight back
land → A flat area between pits on an optical disc that reflects the laser light straight back to the sensor
Data on an optical disc is stored along one long spiral track that starts near the centre and winds outwards while the disc spins → spiral track
A laser shines on the spinning disc, and a sensor detects whether the light is reflected back or scattered, which gives the binary data → laser reflection
A pit is a flat area that reflects the laser back to the sensor → Pits and lands are mixed up. A pit is a dent that scatters the light, while a land is the flat area that reflects it back.
An optical disc stores data in magnets that are read by a laser → Optical discs do not use magnetism. They store data as pits and lands, and the laser reads how light reflects off them.

Key Term

pits and lands
Dents (pits) and flat areas (lands) along a disc's spiral track. A laser reads them by how light reflects: lands reflect it back, pits scatter it. The changes store binary data.

Fill in the Blanks

1. IF surface = PIT THEN light ← SCATTERED ENDIF
Hint: Type what happens to the laser light when it hits a pit.
2. IF surface = LAND THEN light ← REFLECTED ENDIF
Hint: Type the name of the flat area that reflects the laser light straight back.
3. discSpinning ← TRUE trackShape ← SPIRAL
Hint: Type the shape of the single track that winds outwards from the centre of the disc.
4. writing ← TRUE IF writing THEN laser.create(PITS)
Hint: Type the dents that a laser makes on the disc surface when data is written.
5. changes ← DETECT(laser, disc) data ← CONVERT(changes, BINARY)
Hint: Type the number system that the pit and land changes represent.

Spot the Error

Question 1
A disc spins in the drive
B laser shines on the track
C light hits a flat land
D light scatters away
Line D — A land is a flat area, so it reflects the laser light straight back to the sensor. Pits are the parts that scatter light. The line should be: light reflects back to sensor.
Question 2
A laser reads the spiral track
B a land is a flat area
C a pit reflects all the light
D sensor sees less light
Line C — A pit is a dent, so it scatters the laser light instead of reflecting it back. Only lands reflect the light cleanly. The line should be: a pit scatters the light.
Question 3
A WRITE data to optical disc
B laser aims at spiral track
C laser burns pits where needed
D untouched areas become pits
Line D — When writing, the laser makes the pits. Areas it does not touch stay flat, so they remain lands. The line should be: untouched areas stay as lands.
Question 4
A pits are flat areas on a disc
B lands are flat areas
C laser reads reflected light
Line A — Pits are dents in the disc surface. Lands are the flat areas. The line should be: pits are dents in the disc.
Question 5
A disc spins in the drive
B laser moves along the track
C track is concentric rings
D it runs from centre to edge
Line C — An optical disc has one continuous spiral track, not separate concentric rings. The line should be: track is one long spiral.

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