The shape of the lesson
By the end of the lesson, learners can
- Trap light in the fibre: Send a ray into the fibre and watch it bounce along by total internal reflection, even around a bend.
- Why it works, where it is used: The physics is total internal reflection; the applications follow from carrying light with little loss.
- Trapping. Light hits the core boundary above the critical angle, so it is totally internally reflected and stays in the core.
- Drawing. The path is straight line segments meeting the boundary at sharp angles, never a curved or wavy line.
- Uses. Telecommunications (high capacity, low loss) and medical endoscopes.
One picture every idea returns to
An optical fibre guides light along its length by repeated total internal reflection. The light strikes the walls at angles greater than the critical angle, so none escapes. Fibres are used for high-speed communications, where they carry huge amounts of data with little loss, and in medical endoscopes.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Light down a pipe.. A hair-thin thread of glass can carry a phone call, a film, or a surgeon view of your insides, all as pulses of light bouncing thousands of times along its length without ever leaking out. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Trap light in the fibre | Send a ray into the fibre and watch it bounce along by total internal reflection, even around a bend. Project tab 5 of the lab (Optical fibres) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Why it works, where it is used | The physics is total internal reflection; the applications follow from carrying light with little loss. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 14 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Why it works, where it is used, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| Light stays in a fibre because of total internal reflection, not because of any mirror coating. | If a ray meets the wall at less than the critical angle it refracts out and the signal is lost, so the fibre must keep every reflection above the critical angle. |
Optical fibres: worksheet
6 questions, 14 marks. Written in the style of the papers; no past paper question is reproduced.
Explain how an optical fibre keeps a ray of light inside its core as it travels along the fibre.
The cladding glass has a lower refractive index than the core glass. Explain why this is necessary for the fibre to work.
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.
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.
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 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.
Answer key and marking notes
- 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. ✓
- 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. ✓
- 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.
- 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) 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. ✓
- 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. ✓
2 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
How does light travel along an optical fibre?
- Aby repeated total internal reflection along its length
- Bby refracting out through the sides repeatedly
- Cby diffraction around the core
- Dby being absorbed and re-emitted at each end
Optical fibres are widely used in:
- Ahigh-speed communications and medical endoscopes
- Bheating food quickly
- Cgenerating electricity from sunlight
- Dproducing X-ray images
Quiz key
Light is kept inside the fibre by repeated total internal reflection, since it always strikes the walls at more than the critical angle. If it refracted out of the sides (B) the signal would be lost.
Optical fibres carry data as light over long distances in communications and let doctors see inside the body using endoscopes. The other options are unrelated to guiding light along a fibre.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.