Why are the fibres in an endoscope very thin?

A thin fibre can bend sharply without light meeting the boundary below the critical angle. Thin fibres also make a flexible bundle and give finer image detail.

A fibre has a core of refractive index 1.52 and a cladding of refractive index 1.48. What is the critical angle at the boundary?

The critical angle is about 76.8°. The working is sin C = 1.48 ÷ 1.52 = 0.974, so C = 76.8°.

What is a coherent bundle in an endoscope?

A coherent bundle has its fibres in the same relative positions at both ends. Each fibre carries one small part of the image, so the picture arrives intact.

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Medical Physics

Flexible endoscopes, optical fibres and lasers

3 things to know
A2 3

Overview

Total internal reflection and cladding, a worked critical angle, coherent and incoherent bundles, endoscope channels and tools, lasers in surgery, keyhole surgery and cleaning endoscopes.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What is a flexible endoscope?

Key term

How does an optical fibre carry light around bends?

Key term

Why does an optical fibre have a cladding?

Questions from this subtopic

Each one with the model answer underneath.

1
What is a flexible endoscope?
Model answer
A flexible endoscope is a thin tube of optical fibres passed into the body through a natural opening or a small incision. The doctor can view internal organs without open surgery.
2
How does an optical fibre carry light around bends?
Model answer
Light inside the core strikes the core–cladding boundary at more than the critical angle. The light is totally internally reflected again and again along the fibre.
3
Why does an optical fibre have a cladding?
Model answer
The cladding has a lower refractive index than the core, so total internal reflection can occur at the boundary. The cladding also protects the core surface from scratches that would let light escape.
4
Why are the fibres in an endoscope very thin?
Model answer
A thin fibre can bend sharply without light meeting the boundary below the critical angle. Thin fibres also make a flexible bundle and give finer image detail.
5
A fibre has a core of refractive index 1.52 and a cladding of refractive index 1.48. What is the critical angle at the boundary?
Model answer
The critical angle is about 76.8°. The working is sin C = 1.48 ÷ 1.52 = 0.974, so C = 76.8°.
6
What is a coherent bundle in an endoscope?
Model answer
A coherent bundle has its fibres in the same relative positions at both ends. Each fibre carries one small part of the image, so the picture arrives intact.

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