Unit 12 · Medical Physics, Astronomy and Cosmology
The Complete Cambridge International AS & A Level Physics (9702) Guide · Cambridge International AS & A Level Physics (9702)Unit 12 · Medical Physics, Astronomy and Cosmology
- The Complete Cambridge International AS & A Level Physics (9702) Guide
- Cambridge International AS & A Level Physics (9702)
- 12 sections
Cambridge International does not publish a per-unit weighting for this exam. Exam structure (official): AS Level: Multiple Choice (75 min, 40 questions, 31%) · AS Level Structured Questions (75 min, 46%) · Advanced Practical Skills (120 min, 2 questions, 23%); A Level: Multiple Choice (75 min, 40 questions, 15.5%) · AS Level Structured Questions (75 min, 23%) · Advanced Practical Skills (120 min, 2 questions, 11.5%) · A Level Structured Questions (120 min, 38.5%) · Planning, Analysis And Evaluation (75 min, 2 questions, 11.5%). Be ready to carry the same unit skills and representations into a complete answer on every paper. The 130 μs interval covers travel from the transducer to the boundary and back. Multiplying by 1540 m/s therefore gives twice the required depth.
- How Cambridge International AS & A Level Physics (9702) assesses this Cambridge International publishes no per-unit percentage for Cambridge International A Level Physics; Paper 1 accounts for 31% of the AS Level total and 15.5% of the A Level total. · AS Level: Multiple Choice (75 min, 40 questions, 31%) · AS Level Structured Questions (75 min, 46%) · Advanced Practical Skills (120 min, 2 questions, 23%) | A Level: Multiple Choice (75 min, 40 questions, 15.5%) · AS Level Structured Questions (75 min, 23%) · Advanced Practical Skills (120 min, 2 questions, 11.5%) · A Level Structured Questions (120 min, 38.5%) · Planning, Analysis And Evaluation (75 min, 2 questions, 11.5%) · show the model with A graph of depth (m) against return time (μs)., A formula card that explains pulse-echo depth is half the product of propagation speed and measured return time., A table linking key concept, why it matters, visible method.
- Key skills Production and Use of Ultrasound (A Level), Production and Use of X-rays and CT Scanning (A Level), PET Scanning (A Level), Standard Candles, Wien's Displacement Law, the Stefan-Boltzmann Law and Stellar Radii (A Level), Hubble's Law and the Big Bang Theory (A Level)
- How to study for Unit 12 This page turns A graph of depth (m) against return time (μs)., A formula card that explains pulse-echo depth is half the product of propagation speed and measured return time., A table linking key concept, why it matters, visible method. into one route: Convert the microsecond reading to seconds.
- The organizing decision Sketch the outward and return paths so the factor of two comes from geometry rather than from a memorised rule.
What Cambridge International AS & A Level Physics (9702) Unit 12 covers
Use this map to connect each assessed skill to the relationship or representation that makes it visible.
Production and Use of Ultrasound (A Level)
acoustic impedance; Hubble constant; standard candle02Production and Use of X-rays and CT Scanning (A Level)
acoustic impedance; Hubble constant; standard candle03PET Scanning (A Level)
acoustic impedance; Hubble constant; standard candle04Standard Candles, Wien's Displacement Law, the Stefan-Boltzmann Law and Stellar Radii (A Level)
acoustic impedance; Hubble constant; standard candle05Hubble's Law and the Big Bang Theory (A Level)
acoustic impedance; Hubble constant; standard candleCambridge International does not publish a per-unit weighting for this exam.
Official unit name and weighting: Cambridge International syllabus.
Medical Physics, Astronomy and Cosmology
Connect the published share to the unit model
The 130 μs interval covers travel from the transducer to the boundary and back. Multiplying by 1540 m/s therefore gives twice the required depth.
Conversion to 130 × 10⁻⁶ s precedes division by two in d = ct/2. The result 0.1001 m is reported as 0.100 m or 10.0 cm.
A named imaging technique is not a causal explanation by itself. Here the echo time becomes depth only through the propagation speed and pulse-echo geometry.
Identify the measured signal before choosing a propagation, attenuation, geometry or cosmology relation. Make the required time, distance and scale conversions, calculate the requested parameter, and then give the causal link between the measurement and the claimed image, distance, recession or model evidence.
The decision that organizes this unit
Define the system and choose the route before calculating
Sketch the outward and return paths so the factor of two comes from geometry rather than from a memorised rule.
Convert the microsecond reading to seconds
Mechanism route and repair branches
Relationships to preserve
- Measured signal: Return time is 130 μs.
- Geometry: The pulse travels outward and back.
- Depth: 1540 m/s sets the tissue propagation scale.
Representations to read
- A graph of depth (m) against return time (μs).
- A formula card that explains pulse-echo depth is half the product of propagation speed and measured return time.
- A table linking key concept, why it matters, visible method.
Branches to reject
- Using ct as the one-way depth
- Treating microseconds as milliseconds
- Treating the two times as one-way travel
| Key concept | Why it matters | Visible method |
|---|---|---|
| Measured signal | Return time is 130 μs | Convert the microsecond reading to seconds |
| Geometry | The pulse travels outward and back | Divide ct by two |
| Depth | 1540 m/s sets the tissue propagation scale | State 0.100 m or 10.0 cm |
| Measured signal | Echo time becomes depth only through propagation and geometry. | Name the signal and relation before calculating. |
| Causal explanation | Naming an imaging method does not explain the result. | Link the measured return to the reflecting boundary. |
How Cambridge International AS & A Level Physics (9702) assesses Medical Physics, Astronomy and Cosmology
What a complete response must make visible
Match the task to evidence that a reader can audit, then check the most likely reasoning failure before finalizing the response.
| Task | Evidence to show | Hurdle |
|---|---|---|
| Production and Use of Ultrasound (A Level) | acoustic impedance; Hubble constant; standard candle | Using ct as the one-way depth |
| Production and Use of X-rays and CT Scanning (A Level) | acoustic impedance; Hubble constant; standard candle | Treating microseconds as milliseconds |
| PET Scanning (A Level) | acoustic impedance; Hubble constant; standard candle | Treating the two times as one-way travel |
| Standard Candles, Wien's Displacement Law, the Stefan-Boltzmann Law and Stellar Radii (A Level) | acoustic impedance; Hubble constant; standard candle | Using ct as the one-way depth |
| Hubble's Law and the Big Bang Theory (A Level) | acoustic impedance; Hubble constant; standard candle | Treating microseconds as milliseconds |
Worked example: imaging, attenuation, stellar radiation and expansion
Read each question as it appears on the exam paper, choose your answer, then tap it to see the feedback.
Practice 1: Ultrasound pulse-echo depth
An ultrasound pulse reflects from a boundary in tissue.
An ultrasound pulse returns from a boundary after 130 μs. The speed of sound in tissue is 1540 m/s. Determine the boundary depth.
[3]Show all necessary working.
A. d = ct = 0.200 m because the time is for one journey.
❌ Not quite 😭 The stated time covers the outward and return journeys. Using ct gives the total path length, not the depth.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
B. d = ct/2 = 0.100 m because the pulse travels to the boundary and back.
✅ Correct! 😊 Correct. Converting microseconds to seconds and halving the total path gives 0.100 m.
C. d = c/(2t) = 5.92 × 10⁶ m.
❌ Not quite 😭 Dividing speed by time does not give distance and is dimensionally wrong. Distance is speed multiplied by time.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
D. d = ct/2 = 100 m using t = 130 s.
❌ Not quite 😭 This ignores the micro prefix. The measured time is 130 × 10⁻⁶ s, not 130 s.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
Full explanation
- Step 1Convert the time: 130 μs = 130 × 10⁻⁶ s.
- Step 2The pulse travels outward and returns, so ct is twice the boundary depth.
- Step 3Calculate d = ct/2 = (1540)(130 × 10⁻⁶)/2.
- Step 4Report d = 0.100 m, or 10.0 cm, to three significant figures.
Practice 2: Intensity attenuation
Radiation intensity decreases as it passes through material.
The intensity is reduced to 25% of its initial value after passing through two identical layers. Determine the fraction transmitted by one layer.
[3]Show all necessary working.
A. Each layer transmits 12.5% because 25% is divided by two.
❌ Not quite 😭 Successive transmission fractions multiply; they do not add. Dividing the final percentage equally between layers uses the wrong model.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
B. Each layer transmits 50% because the same fraction acts twice and 0.50² = 0.25.
✅ Correct! 😊 Correct. If T is the one-layer fraction, two identical layers give T² = 0.25, so T = 0.50.
C. Each layer transmits 75% because the total loss is 75%.
❌ Not quite 😭 This confuses transmitted fraction with lost fraction. Two layers each transmitting 75% would leave 56.25%, not 25%.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
D. Each layer transmits 25% because identical layers leave the fraction unchanged.
❌ Not quite 😭 A second attenuating layer acts on the already reduced intensity. Repeated layers therefore multiply the transmission factor.
Prep with Sia Agent
Free download for Mac and Windows. Choose your computer:
Full explanation
- Step 1Let T be the fraction of incident intensity transmitted by one layer.
- Step 2Two identical layers transmit T × T = T².
- Step 3Set T² = 0.25 and take the positive square root.
- Step 4Thus T = 0.50, so one layer transmits 50%.
Key terms for Unit 12: Medical Physics, Astronomy and Cosmology
Models, uses, and boundaries
- Medical Physics, Astronomy and Cosmology
- Pulse-echo depth is half the product of propagation speed and measured return time. Use this relationship when pulse-echo depth is half the product of propagation speed and measured return time..
- Worked example: imaging, attenuation, stellar radiation and expansion
- The 130 μs conversion and the round-trip factor lead to a depth of 0.100 m. Use this relationship when the 130 μs conversion and the round-trip factor lead to a depth of 0.100 m..
- Medical Physics, Astronomy and Cosmology: change one condition
- Return times of 100 μs and 150 μs map to depths on the same straight line. Use this relationship when return times of 100 μs and 150 μs map to depths on the same straight line..
- Medical Physics, Astronomy and Cosmology: test proposed results
- A credible depth option must include both the microsecond conversion and division by two. Use this relationship when a credible depth option must include both the microsecond conversion and division by two..
Cambridge International AS & A Level Physics (9702) Unit 12 FAQ
How much of Cambridge International AS & A Level Physics (9702) does Unit 12 carry?
Cambridge International does not publish a per-unit weighting for this exam.
What is the first move on a Medical Physics, Astronomy and Cosmology problem?
Convert the microsecond reading to seconds
Which relationships should I preserve?
Measured signal: Return time is 130 μs. Geometry: The pulse travels outward and back. Depth: 1540 m/s sets the tissue propagation scale.
Which representations should I practice?
Practice moving among A graph of depth (m) against return time (μs)., A formula card that explains pulse-echo depth is half the product of propagation speed and measured return time., A table linking key concept, why it matters, visible method..
What error should I check before submitting an answer?
Check for Using ct as the one-way depth; Treating microseconds as milliseconds; Treating the two times as one-way travel.
Evidence workshop
Continue from the free model into complete practice
The full unit guide continues with the chapter’s worked examples, figures, scoring tables, and answer checks.
- Worked example: imaging, attenuation, stellar radiation and expansion
- Medical Physics, Astronomy and Cosmology: change one condition
- Medical Physics, Astronomy and Cosmology: test proposed results
- Check the worked-example evidence for Medical Physics, Astronomy and Cosmology
Full unit practice. Open the complete guide for the full evidence workshop and synthesis.
Related Cambridge International AS & A Level Physics (9702) unit guides
Cambridge International AS & A Level Physics (9702) Exam Guide & Review
The whole exam and its official unit sequence.01Physical Quantities and Units
Cambridge International does not publish a per-unit weighting for this exam.02Kinematics and Dynamics
Cambridge International does not publish a per-unit weighting for this exam.03Forces, Energy and Deformation of Solids
Cambridge International does not publish a per-unit weighting for this exam.04Waves and Superposition
Cambridge International does not publish a per-unit weighting for this exam.05Electricity and D.C. Circuits
Cambridge International does not publish a per-unit weighting for this exam.06Particle Physics
Cambridge International does not publish a per-unit weighting for this exam.07Circular Motion, Gravitational Fields and Oscillations
Cambridge International does not publish a per-unit weighting for this exam.08Thermal Physics
Cambridge International does not publish a per-unit weighting for this exam.09Electric Fields and Capacitance
Cambridge International does not publish a per-unit weighting for this exam.10Magnetic Fields and Alternating Currents
Cambridge International does not publish a per-unit weighting for this exam.11Quantum and Nuclear Physics
Cambridge International does not publish a per-unit weighting for this exam.How to study Cambridge International AS & A Level Physics (9702) Unit 12
Start with the organizing decision
Before solving, restate the decision in operational terms: Sketch the outward and return paths so the factor of two comes from geometry rather than from a memorised rule.. Your first written move should be to Convert the microsecond reading to seconds.
Practice the same idea in several representations
Rotate through A graph of depth (m) against return time (μs)., A formula card that explains pulse-echo depth is half the product of propagation speed and measured return time., A table linking key concept, why it matters, visible method.. Use each representation to practice Production and Use of Ultrasound (A Level), Production and Use of X-rays and CT Scanning (A Level), PET Scanning (A Level), Standard Candles, Wien's Displacement Law, the Stefan-Boltzmann Law and Stellar Radii (A Level), Hubble's Law and the Big Bang Theory (A Level), and explain what stays invariant when the surface form changes.
Turn each error into a repair check
After every attempt, audit the response for Using ct as the one-way depth; Treating microseconds as milliseconds; Treating the two times as one-way travel. Then redo only the first step that made the reasoning diverge, keeping units, direction, and model conditions visible.
Confirm current course details in the official Cambridge International syllabus for the 2025-2027 administration.
Where to go next
Return to the Cambridge International AS & A Level Physics (9702) to see every unit and plan your revision.