Investigating fibre optic cables

Activity for students aged 14-16 that takes about 60 minutes of class time


Type: Main
Time: 60mins
Subjects: Science, Design & Technology, Engineering
Key words: fibre optics; total internal reflection angle; refraction; reflection; critical angle; perpendicular; material

In this activity students are introduced to the science behind fibre optic cables. They discuss reflection in glass and water and investigate the critical angle for total internal reflection.

semicircular transparent block
light source

Media Resources
Student worksheet



Starter (5 mins)

Ask students to name some properties of light.

Prompt them to state that light travels in straight lines and that light is refracted.

Main (50 mins)

Ask students if they have heard of fibre optic cables, know how they work and why they are useful?

Watch the video from the CHRISTMAS LECTURES which shows the fibre optic cable and total internal reflection of light down the water stream. (5 mins)

Get students to argue why light is a good way to send information. Ask students why light traveling in straight lines may present a problem for optical cables travelling over long distances. (5 mins)

Students investigate the critical angle (40 mins)
Split students into groups and experiment with semicircular clear blocks to find the critical angle. Use the worksheet to help.

Students draw a line along the flat base of the glass block. Shine a light at the curved side of the block and mark the point where the light enters the block, where it leaves the block and another point further away on the lights path. Join the dots to create a diagram of the lights path as it enters and leaves the block as well as a line perpendicular to the base of the block.      

Students must repeat this experiment tracing a diagram of the reflection each time and adjusting the angle that the light leaves the block until the ray is reflected along the flat surface of the block or it is reflected back into the block.

Students then use protractors to measure the angles for each of the diagrams and fill in the table with their results.

Students compare results to find that there is a critical angle greater than which the light will be reflected back into the block.

Plenary (5mins)

Ask students to write a brief description of how a fibre optic cable works using the idea of total internal reflection.

Additional notes

Fibre optic cables are relatively inexpensive and you may even have one in your school. Ask the IT department as it is easy to demonstrate how fibre optic cables transfer light using a torch.

If you do not have sufficient access to semicircular glass blocks you can prisms or gelatin blocks as a medium for measuring the critical angle.

If your students are unable or uncomfortable using a protractor you can get all students to trace and draw their light path and then as a class walk them through using the protractor together. Each students results can be collated into the table and used to find the critical angle.

Do not instruct students on how to measure the angle of incidence. After showing them the diagram tell them the equipment that is available to them and ask them to design a method for measuring the angle of incidence themselves.

Challenge the students to figure out a way without using the protractor to show that the angle of incidence for total internal reflection must be greater than the critical angle [i.e. tracing onto pieces of paper and comparing circular segments]

Curriculum links

UK Science

  • Experimental skills and investigations
    2b make predictions using scientific knowledge and understanding
  • Analysis and evaluation
    3d present reasoned explanations, including explaining data in relation to predictions and hypotheses
  • Sound Waves
    31c sound produced by vibrations of objects, in loud speakers, detected by their effects on microphone diaphragm and the ear drum
  • Current electricity
    35b potential difference, measured in volts, battery and bulb ratings; resistance, measured in ohms, as the ratio of potentidal difference(p.d.) to current

UK Design & Technology

  • Design
    1b identify and solve their own design problems and understand how to reformulate problems given to them
    1c develop specifications to inform the design of innovative, functional, appealing products that respond to needs in a variety of situations
  • Evaluate
    3b evaluate their ideas and products against their own design criteria and consider the view of others to improve their work
  • Technical knowledge
    4a understand and use the properties of materials and the performance of structural elements to achieve functioning solutions

Supported by the Institution of Engineering and Technology


Watch the full lectures

View the full CHRISTMAS LECTURES, Sparks will fly: How to hack your home, along with behind the scenes footage and related content on the Ri Channel