AQA A-Level Computer Science Paper 2, June 2025: Question 3

8 marks · Medium difficulty · Extended Response

Describe the principles of operation of an optical disk including representation and reading of data, and state two reasons why most modern computers lack an optical drive.

Practise this question

Question

Question text

03.1 Describe the principles of operation of an optical disk.

In your response, you should cover how data is represented on a disk and how the

data would be read.

[6 marks]

03.2 State two reasons why most modern computers do not have an optical disk drive.

[2 marks]

Reason 1

Reason 2

Mark scheme

Show the mark scheme

Total

Qu Pt Marking guidance

marks

03 1 Marks are AO1 (understanding) 6

Level Description Mark Range

3 A detailed, coherent, description that covers both the 5–6

reading mechanism and how data is represented,

demonstrating a very good level of understanding.

2 An adequate description, including at least three 3–4

points from the list below. The description may cover

one or both of the reading mechanism and how data

is represented. The description is logically organised

so that it makes sense when read as a whole and

therefore demonstrates a reasonable understanding

of the principles of operation of an optical disk drive.

1 A small number of relevant points have been recalled. 1–2

The structure of the response, or lack of it, means that

only a very limited understanding of the principles of

operation is demonstrated.

Indicative Content

How data is represented

• Data is stored on one/spiral track

A. data is read in a spiral

• Continuation of land/pit reflects light whereas transition between land and pit

scatters light

A. lands reflect light whereas pits scatter light/do not reflect light or vice-versa

A. dye absorbs light whereas absence of dye allows light to pass through/reflect

• Transition between land and pit indicates a 1 and continuation of land/pit

represents 0

A. land represents 1, pit 0 or vice-versa

A. reflection represents 1, no reflection 0 or vice-versa

Accept ‘bump’ for ‘land’ or other wording which clearly reflects nature of

pits and lands

Reading mechanism

• (Low power beam of) laser / light is shone at disk

NE. implication of this because students states that light is reflected back

• Light is focussed on spot on track (A. disk)

• (Some) light is reflected back from disk

• reflected light falls on (light) sensor/photodiode

• Disk spins at constant linear velocity // zoned constant linear velocity // variable

(angular) velocity

– A-LEVEL COMPUTER SCIENCE – –

A. variable speed

R. constant speed

• Head moves in/out to get to particular point on track

Total

Qu Pt Marking guidance

marks

03 2 Marks are AO1 (understanding) 2

(Some) other storage devices have higher storage capacities;

(Some) other storage devices can read/write data more quickly;

Optical drives require a lot of space/are big/are bulky // (some) other storage

devices are more compact;

(Some) other storage devices incorporate the drive and disk/storage;

Data/programs can be downloaded using the Internet // video/music can be

streamed using the Internet // backups can be made to the cloud // use of cloud

storage;

Less content now available on optical disk formats;

(Some) other storage devices are less susceptible to damage (A. more

robust/durable) // optical disks can be easily damaged // optical disks damaged by

scratches;

(Some) type of optical disk cannot be written to // it may only be possible to write

to an optical disk once / a limited number of times;

Award marks if specific other devices (eg SSD, USB flash drive) are named

as the other storage devices, as long as the point made applies to the type

of device.

Max 2

How to answer it

Principles of Optical Storage & Hardware Trends

What This Question Tests

This question examines your in-depth understanding of secondary storage technologies (specification topic: Hardware and Communication).

Physical construction & data representation: Single continuous spiral track, pits vs. lands, reflection scattering, and how transitions represent binary bits.
Reading mechanism: Laser diode, tracking motor, photodiode sensor, and constant linear velocity (CLV).
Technological trends: Evaluating the decline of optical drives in modern computing against solid-state drives (SSDs) and cloud infrastructure.

Question 03.1 • 6 Marks

Operation of an Optical Disk

Describe the principles of operation of an optical disk, including how data is represented and how it is read.

Marking Structure: Levels of Response (AO1 – Understanding)

Level	Marks	Descriptor
Level 3	5 – 6	Detailed, coherent description covering both the reading mechanism and how data is represented, demonstrating a thorough understanding.
Level 2	3 – 4	Adequate description with at least 3 valid points. May focus more heavily on one aspect, but logically structured and sensible overall.
Level 1	1 – 2	A small number of isolated points recalled with limited coherence.

✅ Model Answer (Level 3 Full Marks)

1. How data is represented:

Data is stored along a single continuous spiral track running from the center outwards.
The surface consists of microscopic microscopic indentations called pits and flat areas called lands.
A transition from a land to a pit (or pit to land) causes destructive light interference (scattering) and represents a binary 1.
A continuous flat surface (staying on a land or within a pit) reflects light normally and represents a binary 0.

2. How data is read:

A low-power laser beam is focused by an optical lens onto the track of the rotating disk.
Light reflects off the surface into a photodiode / light sensor.
When the laser hits a transition, the edge scatters the light, causing a drop in detected light intensity; lands/pits reflect steady light.
An actuator arm moves the read head radially (in and out) while a spindle motor spins the disk at a constant linear velocity (CLV) (variable angular speed) so data passes the laser at a uniform rate.

💡 Key Knowledge Breakdown

Representation Nuances:

AQA Mark Scheme Note: While technical standards (like CD-ROM) use transitions = 1 and no transition = 0, examiners also accept:
• Lands = 1, Pits = 0 (or vice versa).
• High reflection = 1, scattering/absorption = 0.
Writable media (CD-R): Uses photosensitive chemical dye instead of physical pressed pits; laser burns spots where dye darkens to absorb light.

Disk Rotation:

CLV: The disk spins faster near the inside and slower near the outside rim to keep the linear track speed over the lens uniform.

🧠 Exam Technique for 6-Mark Questions

Split into subheadings: The question prompt explicitly says: "cover how data is represented on a disk and how the data would be read." Structure your answer with clear headers for both. If you only answer one, you are capped at Level 2 (max 4 marks).
Be precise with physics: Mention the specific detector (photodiode or light sensor) rather than just "the computer sees it".
Explicit motion: Optical disks use a spiral track (unlike magnetic HDDs which use concentric tracks). Mentioning the spiral track immediately establishes high-level knowledge.

❌ Common Errors & Misconceptions

Saying "concentric tracks": Magnetic hard drives have concentric rings; optical disks have one long continuous spiral track.
Writing "constant speed" without qualification: The angular speed varies; only the linear velocity is constant. Writing simply "spins at constant speed" is rejected ( R. constant speed ).
Vague laser descriptions: Saying "the laser burns the disk to read it" confuses read operations with write/burn operations on optical writers. Reading uses a low-power laser that does not alter the medium.

Question 03.2 • 2 Marks

Obsolescence of Optical Disk Drives

State two reasons why most modern computers do not have an optical disk drive.

Marking Guidance (1 mark per distinct point, max 2):
Focus on comparative technical advantages of alternatives (SSDs, internet/cloud) or physical limitations of optical drives.

✅ Any Two Accepted Answers

Physical size / Form factor: Optical drives are bulky and take up substantial internal space; omitting them allows laptops and PCs to be significantly thinner, lighter, and more portable.
High-speed broadband / Cloud distribution: Software, games, music, and OS updates are now downloaded or streamed over the internet / cloud storage, eliminating the need for physical media.
Performance (Speed): Solid-state drives (SSDs) and flash memory provide vastly faster read/write data transfer speeds and lower access times.
Storage Capacity: Optical media has limited capacity (700 MB for CD, 4.7 GB for DVD) compared to modern SSDs and HDDs which offer multiple terabytes.
Durability / Fragility: Optical disks are easily scratched, warped, or damaged by dust and sunlight, leading to read errors.

❌ Avoid Too-Vague Answers

"Nobody uses disks anymore" – Too informal. State that content and software are distributed via digital downloads/streaming.
"They are bad" or "They are outdated" – Must name a specific comparative weakness (e.g. slower transfer rate, lower storage capacity, physical bulk).
"Computers have SSDs now" – Must link this to an actual advantage, such as SSDs have higher capacity, faster access speeds, or smaller form factors.

Topics

4.7 Fundamentals of computer organisation and architecture · 4.7.4 External hardware devices

Question and mark scheme from the AQA A-Level Computer Science examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.