← IGCSE Computer Science Paper 1

Optical discs: CD, DVD and Blu-ray

Optical discs read by laser. Capacity rises CD, DVD, Blu-ray as each laser's shorter wavelength packs pits closer. Blu-ray uses blue-violet. Discs are read-only or recordable.

Part of: IGCSE Computer Science 0478 — 3.3 Data Storage — Module 13

Understanding Optical discs: CD, DVD and Blu-ray in IGCSE Computer Science Paper 1

What it does: A DVD sits in the middle: its red laser is more precise than the CD laser, giving a larger capacity than a CD but less than Blu-ray. ROM discs (like CD-ROM) cannot be changed, whereas R discs (like CD-R) can be written to once by a laser that burns the data in. A CD holds about 700 MB, DVD about 4.7 GB (single layer) and Blu-ray about 25 GB (single layer). CD is the smallest. Blu-ray uses a blue-violet laser. CDs use an infrared laser and DVDs use a red laser. Capacity rises from CD to DVD to Blu-ray as the laser wavelength gets shorter and pits get closer together.

Common mistakes: A CD holds about 700 MB. 25 GB is the capacity of a single-layer Blu-ray disc, so the line should be capacity ← "700 MB". A DVD holds about 4.7 GB because its red laser packs pits closer than a CD. 700 MB is the CD capacity, so the line should be capacity ← "4.7 GB".

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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
Which optical disc has the smallest storage capacity?
CDBlu-ray
Matches
Blu-ray uses a red laser, so it stores more than a DVD
↓
Blu-ray uses a blue-violet laser, not a red one. Its shorter wavelength is what lets it pack pits closer and store more.
Applying
Meanings
Term
CD, DVD and Blu-ray
Meaning
Optical discs read by laser. Capacity rises CD, DVD, Blu-ray as each laser's shorter wavelength packs pits closer. Blu-ray uses blue-violet. Discs are read-only or recordable.
Practice
CD < DVD < Blu-ray
Testing
Gaps
Blu-ray reads with a laser
Check Answer
Spots
2laser ← "infrared"
3capacity ← "25 GB"

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Quick Questions

1. Which optical disc has the smallest storage capacity?
A. CD ✓
B. Blu-ray
A CD holds about 700 MB, DVD about 4.7 GB (single layer) and Blu-ray about 25 GB (single layer). CD is the smallest.
2. Which optical disc type uses a blue-violet laser?
A. Blu-ray ✓
B. DVD
Blu-ray uses a blue-violet laser. CDs use an infrared laser and DVDs use a red laser.
3. Why can a Blu-ray disc store more data than a DVD?
A. Its laser has a shorter wavelength, so pits are packed more closely ✓
B. Its laser is brighter, so the pits are made larger
A shorter wavelength laser can read smaller pits and tracks that are closer together, so more data fits on the same size disc.
4. Which list shows the discs in order of capacity, smallest to largest?
A. DVD, CD, Blu-ray
B. CD, DVD, Blu-ray ✓
Capacity rises from CD to DVD to Blu-ray as the laser wavelength gets shorter and pits get closer together.
5. A CD-ROM or DVD-ROM disc is described as:
A. Recordable: the user can save new data to it
B. Read-only: the data cannot be changed ✓
ROM discs have the pits pressed in at the factory, so you can read them but not write to them. Discs labelled R or RW are recordable.

Match the Pairs

CD (about 700 MB) → The optical disc with the smallest capacity, read by an infrared laser
Blu-ray (25 GB or more) → The optical disc with the largest capacity, read by a blue-violet laser
A DVD holds more than a CD because its red laser has a shorter wavelength, so pits are packed closer together → DVD: red laser, about 4.7 GB
A read-only disc has data pressed in at the factory, while a recordable disc can have data burned onto it by the user → CD-ROM vs CD-R
Blu-ray uses a red laser, so it stores more than a DVD → Blu-ray uses a blue-violet laser, not a red one. Its shorter wavelength is what lets it pack pits closer and store more.
A longer laser wavelength packs pits closer, so capacity increases → A shorter wavelength packs pits closer together. A longer wavelength would mean pits must be spaced further apart and capacity falls.

Key Term

CD, DVD and Blu-ray
Optical discs read by laser. Capacity rises CD, DVD, Blu-ray as each laser's shorter wavelength packs pits closer. Blu-ray uses blue-violet. Discs are read-only or recordable.

Fill in the Blanks

1. Blu-ray reads with a blue-violet laser
Hint: Type the colour of the laser that Blu-ray uses.
2. Capacity order: CD < DVD < Blu-ray
Hint: Type the disc that sits between the smallest and the largest capacity.
3. CD-ROM discs can only be read, not written
Hint: Type the three-letter ending for a disc that is read-only.
4. A shorter wavelength packs pits closer
Hint: Type the word that describes how near together the pits are.
5. Optical discs are read using a laser
Hint: Type the light source that reads the pits and lands.

Spot the Error

Question 1
A disc ← "CD"
B laser ← "infrared"
C capacity ← "25 GB"
Line C — A CD holds about 700 MB. 25 GB is the capacity of a single-layer Blu-ray disc, so the line should be capacity ← "700 MB".
Question 2
A disc ← "DVD"
B laser ← "red"
C capacity ← "700 MB"
Line C — A DVD holds about 4.7 GB because its red laser packs pits closer than a CD. 700 MB is the CD capacity, so the line should be capacity ← "4.7 GB".
Question 3
A disc ← "CD"
B laser ← "blue-violet"
C capacity ← "25 GB"
Line A — The blue-violet laser and 25 GB capacity belong to a Blu-ray disc, not a CD. The line should be disc ← "Blu-ray".
Question 4
A IF disc = "Blu-ray" THEN
B laser ← "red"
C ENDIF
Line B — Blu-ray discs are read with a blue-violet laser. A red laser is used for DVDs. The line should be laser ← "blue-violet".
Question 5
A cd ← 700
B dvd ← 4700
C bluray ← 25000
D top ← MIN(cd, dvd, bluray)
Line D — The capacities (in MB) rise from CD to DVD to Blu-ray, so the largest is found with MAX. MIN would return the smallest (the CD).

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