A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Fibre Optics in Medical Physics — mark scheme explained
The short answer
Fibre optics play a crucial role in medical physics, particularly in the field of endoscopy. This section covers the properties of fibre optics, their applications in medical imaging, and the physical principles behind flexible endoscopes.
The question
A glass fibre has a core with a refractive index of 1.50 and a cladding with a refractive index of 1.46. Calculate the critical angle for total internal reflection at the core-cladding interface.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For questions involving calculations, ensure you show all steps and use the correct formula. For descriptive questions, provide clear and concise answers, including relevant details such as the structure of optical fibres and the principles of TIR. Always explain why certain components or processes are important in medical imaging.
What the command words demand
- Explain
- Provide a detailed account of the principles or processes involved.
- Describe
- Give a detailed account of the structure or function without necessarily explaining why it works that way.
- Calculate
- Perform a mathematical operation to find a specific value, showing all steps clearly.
- List
- Provide a series of items or points without additional explanation.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 5-7 minutes for a 4-mark question to ensure you have enough time to provide a detailed and accurate response.
- Use Snell's Law to find the critical angle: sin(θ c ) = n 2 / n 11 mark
- Substitute the given values: sin(θ c ) = 1.46 / 1.501 mark
- Calculate the critical angle: θ c = arcsin(1.46 / 1.50) ≈ 76.7°1 mark
Final answer: The critical angle for total internal reflection at the core-cladding interface is approximately 76.7°.
Work through every step correctly and you earn all 3 marks.
Another worked example
Explain why coherent fibre bundles are used in endoscopes for image transmission.
- Coherent fibre bundles maintain the spatial arrangement of light, ensuring that high-quality images are transmitted to the viewer.2 marks
- This is crucial for accurate diagnosis and treatment planning.0 marks
Final answer: Coherent fibre bundles are used in endoscopes for image transmission because they maintain the spatial arrangement of light, ensuring that high-quality images are transmitted to the viewer. This is crucial for accurate diagnosis and treatment planning.
Work through every step correctly and you earn all 2 marks.
Common mistakes
Confusing the roles of coherent and non-coherent fibre bundles.
Why it happens: Students may mix up which type of bundle is used for image transmission and which is used for illumination.
Fix: Remember that coherent fibre bundles are used for image transmission because they maintain the spatial arrangement of light, while non-coherent fibre bundles are used for illumination and do not need to maintain a specific spatial arrangement.
Forgetting to mention the critical angle in TIR calculations.
Why it happens: Students may focus on the refractive indices but overlook the importance of the critical angle.
Fix: Always include the calculation of the critical angle using Snell's Law: sin(θ c ) = n 2 / n 1 .
Not explaining how TIR ensures light transmission through the fibre.
Why it happens: Students may describe the structure of the fibre but fail to explain the mechanism of TIR.
Fix: Explain that total internal reflection occurs at the core-cladding interface when the angle of incidence exceeds the critical angle, ensuring that light is guided along the fibre.
Confusing the refractive indices of the core and cladding.
Why it happens: Students may mix up which index is higher or lower.
Fix: Remember that the core has a higher refractive index (n 1 ) than the cladding (n 2 ) to ensure TIR occurs.
Forgetting to mention the jacket in the structure of an optical fibre.
Why it happens: Students may focus on the core and cladding but overlook the protective outer layer.
Fix: Always include the jacket as part of the fibre's structure, noting that it provides physical protection to the fibre.
Not discussing the advantages of using fibre optics in medical imaging.
Why it happens: Students may describe the principles but fail to explain why they are important in practice.
Fix: List and explain at least three advantages, such as minimally invasive procedures, high image quality, and flexibility.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Calculate Critical Angle | Use the refractive indices to find the critical angle at the core-cladding boundary. | 3 |
| Explain Fibre Bundles | Explain why coherent bundles preserve spatial order to transmit clear images in endoscopes. | 2 |
| Optical Fibre Structure | Describe the fibre's core and cladding and explain how they enable total internal reflection. | 4 |
| State Advantages | List three benefits of using fibre optics in medical imaging applications. | 3 |
| Explain Fibre Function | Describe how non-coherent bundles carry light to illuminate the area being examined. | 2 |
| Total across these question types | 14 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.