A student is investigating how light behaves when it travels from glass into air. When the angle of incidence inside the glass is exactly equal to the critical angle, the refracted ray travels along the boundary. Explain what happens to the light ray when the angle of incidence is increased beyond the critical angle, and explain why optical fibres rely on this process to transmit data over long distances.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Optical fibres are thin strands of glass used to carry light signals over long distances. Light enters one end of the fibre and must reach the other end without escaping through the sides of the glass.
Model answer (5 marks)
When the angle of incidence exceeds the critical angle, total internal reflection occurs – all of the light is reflected back into the glass and none escapes into the air. The reflected ray obeys the law of reflection, so the angle of reflection equals the angle of incidence.
In an optical fibre the core is a denser glass surrounded by a less dense cladding (air or a lower‑index material). Light launched into the core strikes the core–cladding interface at an angle greater than the critical angle, so it is totally internally reflected. The light therefore bounces repeatedly along the inside of the fibre, remaining confined to the core and travelling the full length of the fibre without loss of signal.
Thus optical fibres rely on total internal reflection to transmit data over long distances.
In an optical fibre the core is a denser glass surrounded by a less dense cladding (air or a lower‑index material). Light launched into the core strikes the core–cladding interface at an angle greater than the critical angle, so it is totally internally reflected. The light therefore bounces repeatedly along the inside of the fibre, remaining confined to the core and travelling the full length of the fibre without loss of signal.
Thus optical fibres rely on total internal reflection to transmit data over long distances.
Examiner tips
- Use the term "total internal reflection" and state that all light is reflected back into the denser medium.
- Mention that the reflected ray obeys the law of reflection (angle of incidence = angle of reflection).
- Explain that the core–cladding interface has a lower‑index material so the incident angle is always above the critical angle.
- Show how this confinement allows the light to travel the entire length of the fibre without escaping.
Mark scheme (5 marks)
- When the angle of incidence is greater than the critical angle, total internal reflection occurs
- All of the light is reflected back into the glass / denser medium (none escapes into the air)
- The angle of reflection equals the angle of incidence (law of reflection applies)
- In an optical fibre, light travels from a denser medium (glass) to a less dense medium (air/cladding), so the angle of incidence inside the fibre exceeds the critical angle at the glass–air boundary
- Total internal reflection keeps the light bouncing along the inside of the fibre, allowing it to travel the full length without escaping / losing signal
Key terms in this question
critical angle · optical fibre
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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