AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Properties of fibre optics and applications in medical physics; including…
Full specification wording
Fibre optics and endoscopy (A-level only): Properties of fibre optics and applications in medical physics; including total internal reflection at the core–cladding interface.
An optical fibre traps light by total internal reflection (TIR) at the boundary between a core of higher refractive index and a cladding of lower refractive index.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
An optical fibre traps light by total internal reflection (TIR) at the boundary between a core of higher refractive index and a cladding of lower refractive index. Light entering within the acceptance angle strikes the core–cladding interface at an angle greater than the critical angle, so it reflects repeatedly and travels along the fibre with little loss. The critical angle C satisfies sin C = n_cladding / n_core for light going from core to cladding. In medical physics, fibre bundles carry light into the body and return images or signals, enabling endoscopy, illumination and some sensing applications without large incisions.
What earns the marks
- Total internal reflection occurs when light travels from a medium of higher refractive index to one of lower refractive index and the angle of incidence exceeds the critical angle.
- The critical angle C is given by sin C = n_cladding / n_core, where n_core > n_cladding.
- Light must enter the fibre within the acceptance angle so that it strikes the core–cladding boundary at an angle greater than C.
- At each TIR, the angle of reflection equals the angle of incidence and the light remains inside the core, so the signal can travel along a curved fibre.
- In medical physics, fibre optics allow illumination and imaging inside the body, for example in endoscopes, with reduced need for open surgery.
Common mistakes and how to put them right
Thinking that TIR can happen when light travels from a lower to a higher refractive index.
Put it right: TIR requires travel from higher to lower refractive index, as from core to cladding.
Using sin C = n_core / n_cladding.
Put it right: For core-to-cladding TIR, sin C = n_cladding / n_core.
Believing that any ray entering the fibre will be trapped.
Put it right: The ray must enter within the acceptance angle so that it meets the interface above the critical angle.
Exam tips
- Sketch the core and cladding, mark the normal at the interface, and label the angle of incidence and the reflected ray to show TIR clearly.
- When calculating the critical angle, state the refractive indices used and check that the value is less than 90°.
- Link each property of the fibre to a medical use, such as flexible bundles for imaging or illumination during endoscopy.
Try the lesson
The start of the Fibre optics and endoscopy (A-level only): Properties of fibre optics and applications in medical physics; including total internal reflection at the core–cladding interface. lesson, made from checked teaching notes for this specification point.
Have a go first
In an optical fibre, light is confined to the core by total internal reflection. Which condition must be satisfied at the boundary between the core and the cladding for total internal reflection to occur?
Listen
An optical fibre traps light by total internal reflection (TIR) at the boundary between a core of higher refractive index and a cladding of lower refractive index. Light entering within the acceptance angle strikes the core–cladding interface at an angle greater than the critical angle, so it reflects repeatedly and travels along the fibre with little loss. The critical angle C satisfies sin C = n_cladding / n_core for light going from core to cladding. In medical physics, fibre bundles carry light into the body and return images or signals, enabling endoscopy, illumination and some sensing applications without large incisions. A strong answer earns marks for these points. Total internal reflection occurs when light travels from a medium of higher refractive index to one of lower refractive index and the angle of incidence exceeds the critical angle. The critical angle C is given by sin C = n_cladding / n_core, where n_core > n_cladding. Light must enter the fibre within the acceptance angle so that it strikes the core–cladding boundary at an angle greater than C. At each TIR, the angle of reflection equals the angle of incidence and the light remains inside the core, so the signal can travel along a curved fibre. A common slip: Thinking that TIR can happen when light travels from a lower to a higher refractive index. Instead, TIR requires travel from higher to lower refractive index, as from core to cladding. Exam tip: Sketch the core and cladding, mark the normal at the interface, and label the angle of incidence and the reflected ray to show TIR clearly.
Key words
Say what each word means before you turn the card.
Total internal reflection
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Critical angle and refractive index
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Optical fibres
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Medical imaging and endoscopy
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See every definition
- Total internal reflection
- Occurs when light travels from a medium of higher refractive index to one of lower refractive index and the angle of incidence exceeds the critical angle.
- Critical angle and refractive index
- The critical angle C is given by sin C = n_cladding / n_core, where n_core > n_cladding.
- Optical fibres
- Light entering within the acceptance angle strikes the core–cladding interface at an angle greater than the critical angle, so it reflects repeatedly and travels along the fibre with little loss.
- Medical imaging and endoscopy
- Fibre bundles carry light into the body and return images or signals, enabling endoscopy, illumination and some sensing applications without large incisions.
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