📞 Communications
Broadband internet and telephone signals travel as pulses along optical fibres under streets and across the sea bed. The pulses stay inside the glass, so the signal arrives clear and strong after a long distance.
Light and Sound » Optical Fibres and Prisms
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 3.20
- How total internal reflection keeps light inside an optical fibre
- How optical fibres carry information, and how an endoscope works
- How a right-angled prism turns light through 90° and 180°
- Where prisms are used: periscopes, binoculars and bicycle reflectors
🔒 Unlock Full Course Content
Sign up to access the complete lesson and track your progress!
Unlock This CourseTrapping light inside glass
Remember, light inside glass that meets the boundary at more than the critical angle is totally internally reflected. None of it escapes. This lesson is about two ways that idea is put to work: optical fibres and prisms.
In both, the glass acts as a perfect mirror with no shiny coating at all. For ordinary glass in air the critical angle is about 42°, so any ray that meets the inside surface at more than 42° from the normal is reflected back in.
Key terms:
- Optical fibre: a very thin, flexible strand of glass that carries light from one end to the other by total internal reflection.
- Pulse: a short flash of light or infrared. A pattern of pulses is a signal that carries information.
- Endoscope: a medical instrument that uses bundles of optical fibres to see inside the body.
- Right-angled prism: a triangular glass block with angles of 45°, 45° and 90°. It has two short faces and one long face.
- Periscope: an instrument for seeing over or around an obstacle.
How an optical fibre works
An optical fibre is about as thin as a human hair. Light is sent in at one end. Here is the path of one ray, step by step:
- The ray enters the end of the fibre and travels through the glass.
- It meets the inside wall of the fibre. Because the fibre is so thin and the ray is travelling almost along it, the angle of incidence is large, well above the critical angle.
- The ray is totally internally reflected. No light leaves through the wall.
- The reflected ray crosses the fibre and meets the opposite wall, again at more than the critical angle. It is totally internally reflected again.
- This happens again and again, so the ray zigzags along the fibre until it comes out of the far end.
Because the reflection is total each time, very little light is lost, even after thousands of reflections. The fibre can also curve gently and the light still follows it, because each ray still meets the wall at more than the critical angle.
To draw it: draw two long parallel lines close together for the walls of the fibre. Draw a ray entering at the left-hand end and meeting the top wall. Reflect it down to the bottom wall, then up again, in a zigzag of straight lines. At every reflection the angle of reflection must look equal to the angle of incidence. Put arrows on the ray and show it leaving at the right-hand end. No ray should pass through a wall.
Worked example 1
Question: A ray inside a glass fibre meets the wall at an angle of incidence of 75°. The critical angle is 42°. Describe what happens to the ray.
75° is greater than the critical angle of 42°.
So the ray is totally internally reflected and no light leaves through the wall.
The angle of reflection is 75°, so the ray carries on along the fibre and meets the opposite wall.
Transmitting information along optical fibres
To send information, a transmitter at one end turns it into a pattern of pulses of light or infrared. The pulses travel along the fibre by total internal reflection. A detector at the far end turns the pattern back into sound, pictures or data.
🏥 Endoscopes
An endoscope lets a doctor see inside the body, for example inside the stomach, without a large cut. It contains two bundles of fibres. One bundle carries light down to light up the inside of the body. The other carries the reflected light back up, so an image can be seen.
In an endoscope, each fibre in the second bundle carries the light from one tiny part of the scene. Together they build up the whole picture. The fibres are thin and flexible, so the endoscope can bend around corners inside the body.
Turning light with a right-angled prism
A right-angled glass prism can reflect light with no mirror. It works because of one simple fact: the ray meets the inside of the glass at 45°, and 45° is bigger than the critical angle of glass, about 42°. So the ray is totally internally reflected.
📐 Turning through 90°
1. The ray enters one short face along the normal, so it does not bend.
2. It meets the long face from inside at an angle of incidence of 45°.
3. 45° is greater than 42°, so it is totally internally reflected. The angle of reflection is 45°.
4. It leaves through the other short face along the normal, so it does not bend. The ray has turned through 90°.
🔄 Turning through 180°
1. The ray enters the long face along the normal, so it does not bend.
2. It meets one short face at 45° and is totally internally reflected, turning through 90°.
3. It crosses the prism, meets the other short face at 45° and is totally internally reflected again, turning through another 90°.
4. It leaves through the long face along the normal. The ray now travels back the way it came, on a parallel path.
To draw them: draw the triangle with a small square in the 90° corner. Draw rays with a ruler and add arrows. Where the ray meets a face from inside, draw a dashed normal and make the two 45° angles look equal. Rays crossing a face along the normal go straight through with no bend.
Where prisms are used
| Device | What the prisms do |
|---|---|
| Periscope | Two prisms, one at the top of a tube and one at the bottom. Each turns the light through 90°. Light enters at the top, travels down the tube and out to the eye, so you can see over an obstacle. |
| Binoculars | Each side has two prisms. Each prism turns the light through 180°. This folds a long light path into a short tube. |
| Bicycle reflector | The back of the red plastic is moulded into many tiny prism shapes. Light from a car's headlamps is totally internally reflected twice and sent straight back towards the driver. |
A prism has an advantage over a mirror here. Total internal reflection reflects all of the light, so the image is bright.
Worked example 2
Question: A right-angled prism is cut from clear ice, which has a refractive index of 1.31. A ray enters a short face along the normal. Explain whether the prism turns the ray through 90°.
sin c = 1 ÷ n = 1 ÷ 1.31 = 0.763
c = sin−1(0.763) = 49.8°
The ray meets the long face at 45°. This is less than the critical angle of 49.8°, so the ray is not totally internally reflected. Most of the light refracts out through the long face.
The prism only works if the critical angle of the material is less than 45°. Glass, at about 42°, is suitable.
Common mistakes
- Writing that light is 'reflected' along a fibre. Say totally internally reflected, and say the angle of incidence is greater than the critical angle.
- Saying the light bends or curves along the fibre. It travels in straight lines between reflections.
- Drawing a ray that bends as it enters a prism along the normal. A ray along the normal goes straight on.
- Saying a prism works because 45° equals the critical angle. It works because 45° is greater than the critical angle of about 42°.
- Saying an endoscope has one bundle of fibres. It needs two: one to carry light in and one to carry the image out.
Exam-style question
Optical fibres and prisms both use total internal reflection.
(a) Describe how a pulse of light travels from one end of an optical fibre to the other. [3 marks]
(b) State one use of optical fibres in medicine. [1 mark]
(c) A ray of light enters one short face of a right-angled glass prism along the normal. The critical angle of the glass is 42°.
(i) State why the ray does not change direction as it enters the prism. [1 mark]
(ii) Explain what happens to the ray when it meets the long face of the prism. [3 marks]
(iii) State the angle through which the prism turns the ray. [1 mark]
(d) Name one device that uses prisms to turn light in this way. [1 mark]
Model answer
(a) The light meets the inside wall of the fibre at an angle of incidence greater than the critical angle (1). It is totally internally reflected (1). This happens again and again along the fibre until the light leaves at the far end (1).
(b) In an endoscope, to see inside the body (1).
(c)(i) It travels along the normal, so the angle of incidence is 0° (1).
(c)(ii) The angle of incidence at the long face is 45° (1). This is greater than the critical angle of 42° (1). So the ray is totally internally reflected, with an angle of reflection of 45° (1).
(c)(iii) 90° (1).
(d) A periscope (1).
Exam tip
In parts (a) and (c)(ii), the marks are for the comparison. Always write that the angle of incidence is greater than the critical angle before you write 'total internal reflection'. In (c)(ii), quote both numbers: 45° and 42°.