1. By the end of this presentation, you should be able to: Recognise diagrams showing TIR Recall what conditions must be met in order for TIR to occur. Draw diagrams to illustrate TIR of light in glass. Describe applications of TIR.

2. Total Internal Reflection Occurs only if the light is passing into a less dense material. So it cannot happen if the ray is travelling from air into glass (or water). Happens only at certain angles ( when the light hits the glass (or water) surface at an angle of incidence that is greater than the critical angle.

3. Total internal reflection of light in water When the ray hits at 90 o, it passes straight out into the air.As the angle of incidence (in water) is increased, refraction is observable. If the angle of incidence (in water) is too large, refraction will not be allowed and the wave is reflected internally. i

4. TOTAL INTERNAL REFLECTION Look carefully at the following slides. It shows the stages leading up to total internal refraction.

5. TIR only takes place when light travels into a less dense material, eg, from glass into air This angle of incidence is very small. The ray is refracted as usual normal r i

6. As you increase the angle of incidence in glass, the angle of refraction in the air will increase too.

7. When the angle of incidence has increased so much that the angle of refraction is 90 o, you have reached a special angle. When the angle of refraction is 90 o, the special angle of incidence is called the critical angle. r =90

8. If the angle of incidence is increased any more, so that it is larger than the critical angle, total internal reflection occurs. i > c

9. Light turns through 90 o. Used in periscopes. Light turns through 360 o. Used for bicycle reflectors and cats eyes in the road.

10.  Information is sent along optic fibres in communication systems. It has the advantage of being more secure than copper wires, and more information can be sent along one cable.  Optic fibres can be used in medicine as well. Endoscopes can be inserted into small incisions, or can be pushed down the throat so the inside of the body can be seen without surgery.

11. A fibre optic is a very narrow length of glass or plastic. The light shines in at one end of the fibre and does not emerge until it reaches the other end. The light (or infra-red signal) is totally internally reflected a number of times inside the cable. It cannot be seen until it leaves the fibre.

12.  Optical fibres are replacing copper wires used in telephone systems.  Telephone conversations are sent down the optical fibre by switching on and off a LED (Light Emitting Diode), a laser or infra-red rays.  At the other end, the signals are converted back to electricity. The signals are digital.  Over 10 000 telephone conservations can be carried at the same time by one fibre! How? Multiplexing. Advantages of optical fibres over copper cables:  Thinner  Cheaper  Carry more signals  No cross-talk between phone conversations  Difficult to ‘bug’.

13. Lasers can send millions or billions of signals of pulses down an optical fibre every second. So a fibre can carry millions of calls at the same time.  The message is divided up into a series of batches or pulses.  The laser flashes the first batch of your message, then a batch from someone else’s, then a batch from a third message.  Eventually it gets back to your message and sends the next batch.  At the other end of the fibre is a processor which separates out the batches and recombines them into the correct sequence.  This is why you can use one phone line for phone calls and broadband internet.  It is very unlikely that one signal will interfere with another – no signals can get in from outside and no signals can leak outside.

14. Uses of optical fibres - Endoscopes Reflected light comes back up the bundle of fibres to form an image of bright and dark spots of light. Doctors use optical fibres to look at the inside of people’s lungs and stomachs: A large number of optical fibres are held together in a bundle which goes down the patient’s throat. Light is sent down some of the fibres to illuminate the stomach.