AQA A-Level Computer Science AS Paper 2, June 2025: Question 11

✅ Mark Scheme Points (Any 3 for 3 marks)

• Each cell / photosensor produces an analogue voltage proportional to the amount/brightness of light falling on it (or converts photons into an accumulated electrical charge).
• An Analogue-to-Digital Converter (ADC) converts these analogue voltages into a digital format / binary.
• Colour filters (red, green, blue) are placed over cells to capture individual colour channels/components.
• Use of a CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor) sensor array.

💡 Key Knowledge: The Capture Pipeline

1. Filtration: Light passes through a Bayer filter (RGB grid) so each sensor cell captures a specific colour.
2. Photoelectric Effect: Photons strike the CMOS/CCD photosite, freeing electrons to create an electrical charge/voltage.
3. Quantisation: The continuous analogue voltage is sampled and converted by an ADC into discrete binary values (e.g. 0 to 255 for 8-bit channels).

🧠 Exam Technique: Focus on the Word "After"

The prompt explicitly begins: "after light has been focused on the light-sensitive cells".

• Do not waste time describing lenses, apertures, or the shutter opening—no marks are awarded for events before light hits the sensor.
• Always follow the physical-to-digital pathway: Light → Charge/Voltage → ADC → Binary. Mentioning the ADC is an easy, high-frequency mark on AQA papers.

❌ Common Errors

• Vague electrical terms: Saying "it turns light into electricity" without mentioning analogue voltage or accumulated charge.
• Missing the ADC: Stating the sensor "immediately reads 1s and 0s". Light sensors are inherently analogue devices; an ADC is strictly required.
• Omitting colour: Forgetting that sensors only detect light intensity unless RGB filters are used.

✅ Mark Scheme Points (Any 3 for 3 marks)

• Data is stored in floating gate transistors (or transistors that retain charge/state when power is removed / electrons trapped between oxide layers).
• The presence or absence of trapped charge represents a binary state (0 or 1).
• Utilises NAND flash memory (or EEPROM technology).
• Data is structured and organised into pages and blocks.
• Data cannot be overwritten directly: an entire block must be erased before being rewritten.
• An onboard controller manages read, write, and wear-levelling operations.

💡 Key Knowledge: How Floating Gates Work

• Structure: A floating gate is completely insulated by thin silicon oxide layers between the control gate and the substrate channel.
• Tunneling (Writing): High voltage pushes electrons across the oxide barrier (Fowler-Nordheim tunneling), trapping them in the floating gate.
• Reading: Trapped electrons alter the threshold voltage required to turn the transistor channel on, distinguishing a logical 0 from a logical 1.
• Non-volatility: Because the gate is electrically isolated, the charge remains trapped for years without power.

🧠 Exam Technique: Be Precise with Physics

• Say "presence or absence of trapped electrons/charge represents 0 or 1" rather than vague statements like "positive and negative charges represent bits".
• Remember the hierarchy: Bits → Cells → Pages → Blocks. Writes happen at the page level, but erasing can only happen at the block level.

❌ Common Misconceptions to Avoid

• Reject: "Positive and negative charges": Mark schemes explicitly state Reject: positive and negative charges. It is about the presence vs. absence (or threshold shift) of charge.
• Confusing SSD with HDD or Optical: Never refer to magnetic domains, tracks, sectors, lands, pits, or lasers.
• "Stores binary": Saying "it stores binary" gains no credit; you must state that the state represents a 0 or 1 bit.