Home · IGCSE 0625 · Light · Optical fibres · Practice
Extended · Practice questions · Optical fibres

Straight lines. Sharp bounces.

Six original Cambridge-style questions. They cover how a fibre traps light, why the ray must be drawn as straight ruler-lines, and where fibres are used in communication and medicine.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
What earns the marks here

Total internal reflection, drawn with a ruler.

01
[3 marks]

Explain how an optical fibre keeps a ray of light inside its core as it travels along the fibre.

  • Light strikes the boundary between the core and the cladding at an angle of incidence greater than the critical angle. ✓
  • So the light is totally internally reflected at the boundary. ✓
  • It reflects repeatedly along the fibre, staying inside the core the whole way. ✓
02
Analysis
[2 marks]

The cladding glass has a lower refractive index than the core glass. Explain why this is necessary for the fibre to work.

  • Total internal reflection only happens when light travels toward a less dense (lower refractive index) medium. ✓
  • The lower-index cladding gives the core a critical angle, so light hitting the boundary steeply enough is totally internally reflected instead of escaping. ✓

If the cladding had a higher index, light would refract out and the signal would leak away.

03
Diagram
[2 marks]

The diagram shows a ray of light entering the core of a straight optical fibre. On the diagram, complete the path of the ray until it leaves the far end of the fibre.

cladding cladding core ray in
Complete the ray path to the end of the fibre.
cladding cladding ray in
Straight segments, total internal reflection at each boundary.

Straight ruler-drawn lines, reflecting at the core boundary each time, with the angle of reflection equal to the angle of incidence. ✓✓

No curves, no wiggles. Each bounce is a sharp, clean reflection.

04
Analysis
[2 marks]

In an exam, a student completes the ray path inside an optical fibre by drawing a smooth, curved, wavy line winding down the centre of the core. Explain why this drawing would not earn the marks, and state what the correct drawing should look like.

  • Light does not curve. Between reflections it travels in straight lines, so a wavy line is physically wrong. ✓
  • The correct path is a zig-zag of straight, ruler-drawn segments that reflect off the core-cladding boundary at sharp angles. ✓

A curved or "hairy" line is a guaranteed lost mark. Reach for a ruler.

05
[3 marks]

Optical fibres have largely replaced copper wires for long-distance communication.

(a) State two uses of optical fibres in carrying information. [2] (b) State one advantage of an optical fibre over a copper wire for carrying signals over a long distance. [1]

(a) Any two of: telephone calls, internet data, cable television signals. ✓✓

(b) Any one of: carries far more information at once (higher capacity); loses very little signal strength over distance (low loss); or is not affected by electrical interference. ✓

06
Analysis
[2 marks]

A doctor uses an endoscope, a bundle of optical fibres, to see inside a patient's stomach without surgery. Explain how total internal reflection makes this possible.

  • Light is sent down one set of fibres into the body, staying trapped by total internal reflection even as the flexible fibres bend around inside. ✓
  • Light reflected from inside the body travels back up another set of fibres, again by total internal reflection, to form an image the doctor can see. ✓

The fibres bend, but the light inside still travels in straight segments between reflections.

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.