A scientist is designing an optical fibre made from a dense glass material. Light travels along the fibre by entering the glass from air, passing through the glass, and then striking the inside surface of the glass at a very shallow angle. Explain what happens to the light at each stage of this journey, including why the light stays inside the glass when it strikes the inside surface at a very shallow angle.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Optical fibres are used in medicine and telecommunications to carry information as pulses of light over long distances without the light escaping through the sides of the fibre.
Model answer (5 marks)
When light enters the fibre from air it slows down because the refractive index of glass is higher than that of air. The change in speed causes the ray to bend towards the normal – this is refraction.
Inside the glass the ray travels until it meets the inner surface. Because the glass is denser than the surrounding air, the angle of incidence (measured from the normal) is very large – it exceeds the critical angle for the glass–air interface.
At this point total internal reflection occurs: all of the light is reflected back into the glass rather than refracting into the air. Total internal reflection can only happen when light goes from a medium of higher refractive index to one of lower refractive index, i.e. from glass to air.
Inside the glass the ray travels until it meets the inner surface. Because the glass is denser than the surrounding air, the angle of incidence (measured from the normal) is very large – it exceeds the critical angle for the glass–air interface.
At this point total internal reflection occurs: all of the light is reflected back into the glass rather than refracting into the air. Total internal reflection can only happen when light goes from a medium of higher refractive index to one of lower refractive index, i.e. from glass to air.
Examiner tips
- Show the change in speed and bending at the first interface; mention refractive index. Explain that the large angle of incidence is > critical angle. State that total internal reflection keeps the light inside. Remember the condition: denser to less dense medium.
Common mistakes
- Forgetting to mention the slowing of light or the refractive index. Confusing the direction of the normal or the angle of incidence. Saying the light escapes instead of reflecting back.
Mark scheme (5 marks)
- When light passes from air into the glass it changes speed (slows down)
- The light bends towards the normal as it enters the glass (refracts towards the normal)
- When striking the inside surface at a very shallow angle, the angle of incidence (measured from the normal) is large / greater than the critical angle
- Total internal reflection occurs — all the light reflects back into the glass
- Total internal reflection can only occur when light travels from a denser medium to a less dense medium (glass to air)
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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