On this page
Doctors today can look inside a patient's stomach through a thin, bendable tube. Light travels down glass fibres and an image comes back up. In 1954 that was a new idea, and one of the two physicists who first made it work was Narinder Singh Kapany, then working at Imperial College London. This post sets out what the record shows about his work, explains how a fibre bundle carries a picture, and works through the acceptance angle of a fibre.
What did Hopkins and Kapany publish in 1954?
On 2 January 1954 the journal Nature printed A flexible fibrescope, using static scanning by H. H. Hopkins and N. S. Kapany of the Department of Physics, Imperial College of Science and Technology, London. The abstract describes an optical unit that conveys images along a flexible axis. It is a bundle of fibres of glass or other transparent material, and the authors wrote that "it therefore appears appropriate to introduce the term 'fibrescope'". Its obvious use, they said, was to replace the train of lenses in endoscopes such as gastroscopes, which were rigid or only slightly flexible, gave poor images and could contain as many as fifty lenses.
In the same issue, the Dutch physicist A. C. S. van Heel described a related method. His abstract reports that coating each fibre with a layer of lower refractive index, which ensures total reflection, worked much better than coating with metal. That lower-index coating, the cladding, is what every optical fibre uses today.
How did the work continue?
In February 1955 Hopkins and Kapany reported further experiments in Optica Acta. The abstract describes a new machine for making bundles 75 cm long from fibres of about 25 micrometres diameter, straightened so that all the fibres have equal length, and measurements of the resolution the bundles could achieve. Kapany kept working on the idea. In Nature he reported high-resolution fibre optics using sub-micron multiple fibres in 1959, a hypodermic probe using fibre optics in 1961 and a fibre optics spectrophotometer for oximetry in the body in 1964. He also wrote the article "Fiber Optics" for Scientific American in November 1960.
On 25 January 2021 the Government of India announced the Padma Awards, with Narinder Singh Kapany among the seven Padma Vibhushan awardees, posthumous, in science and engineering, listed under the United States of America.
How does a bundle of fibres carry a picture?
Each fibre traps light by total internal reflection. The NCERT Class 12 Physics chapter on ray optics explains that a fibre has a core of higher refractive index than its cladding, so light striking the boundary beyond the critical angle reflects again and again, even when the fibre bends. NCERT adds that such fibres are used as a "light pipe" to examine internal organs like the stomach and intestines.
One fibre carries only brightness, not a picture. A bundle carries a picture if every fibre stays in the same position at both ends, so that each fibre carries one dot of the image. That is the "static scanning" in the 1954 title, and it is why the 1955 work cared about making fibres of equal length in an orderly bundle.
Worked example: how wide a cone does a fibre accept?
The values below are sample numbers. A fibre has a core index n₁ = 1.62 and a cladding index n₂ = 1.52, and sits in air.
Critical angle at the core-cladding boundary. sin C = n₂/n₁ = 1.52/1.62 = 0.938, so C ≈ 69.8°.
Numerical aperture. NA = √(n₁² − n₂²) = √(2.6244 − 2.3104) = √0.314 ≈ 0.560.
Acceptance angle. In air, sin θ = NA, so θ ≈ 34.1°. Rays entering the end within 34° of the axis are trapped; steeper rays leak out.
Why cladding matters. With no cladding, n₂ = 1 (air) and √(n₁² − 1) = 1.27, above 1, so every ray is accepted. But wherever two bare fibres touch, light leaks from one to the other and the picture blurs. A cladding keeps each fibre's light to itself.
Image detail. With fibres 25 µm across, a 1 mm line holds 1/0.025 = 40 fibres. A square bundle 5 mm wide holds (5/0.025)² = 200 × 200 = 40,000 fibres, so 40,000 dots of picture.
Common mistakes with fibre questions
Swapping the indices. n₂/n₁ must be less than 1, with the core index on the bottom.
Measuring from the surface. The critical angle is from the normal to the boundary; the acceptance angle is from the fibre axis.
Forgetting the outside medium. sin θ = NA holds in air; in water divide by 1.33.
Thinking one fibre makes an image. It carries light, not a picture.
Total internal reflection and its applications are in the optics unit of the JEE (Main) 2026 syllabus; check the current year's version. For more practice, use our guide to total internal reflection and critical angle problems. For the story of fibres that carry signals rather than pictures, read about Charles Kao and the glass fibres that carry the internet. If you are not sure where to start in optics, Fermi's AI tutor can work out what you already know and build a plan for the session.
What can a student take from this work?
Start from a real need. The 1954 paper begins with the problems of rigid gastroscopes.
Name what you make. "Fibrescope" gave the idea a handle others could use.
Keep building. Kapany's later papers moved from finer fibres to probes and measurements inside the body.
Practice routine: find the NA and acceptance angle for n₁ = 1.50 and n₂ = 1.46, find how many 10 µm fibres fit in a 3 mm square bundle, and explain why fibres must keep their order.
Next step: draw one fibre with core and cladding, and mark the critical angle and the acceptance cone on it.
