KS4 Waves: Total Internal Reflection and Communication LEARNING OUTCOMES: Describe what happens to light incident on a perspex/glass-air surface both above and below the critical angle of incidence and describe how light is reflected at the inner face of a right-angled prism. Explain how optical fibres are used in endoscopy. Explain how optical fibres allow the rapid transmission of data using digital signals.

Total Internal Reflection Angle r Refracted ray Angle r Angle i Incident ray Reflected ray At what angle of incidence did the ray change from refraction to reflection? It depends upon the material you used.

Simulation of a ray of light passing through a semi-circular glass block

Total Internal Reflection This angle is called the critical angle [ c]  i <  c Refraction  i =  c Critical case  i >  c Total Internal Reflection [TIR] Different materials have different critical angles. Diamond has the lowest at 24º which is why it reflects so much light.

The Critical Angle Material Refractive index Critical angle Glass How does refractive index affect the critical angle? Research the missing values below and then make a conclusion… Material Refractive index Critical angle Glass Water Diamond 1.5 42° 1.33 49° 2.4 24° The greater the refractive index the smaller the critical angle.

Total Internal Reflection Optical fibres, that are used in communication, use total internal reflection. What are the applications of total internal reflection? You could be asked to draw on the path of the beam in an exam. A beam of light enters the optical fibre…. …NOTE how it is refracted as it enters the fibre… …it travels down the fibre through repeated total internal reflections.

Angle of incidence the SAME as critical Angle of incidence LESS than critical Angle of incidence GREATER than critical results in TOTAL INTERNAL REFLECTION

Angle of incidence LESS than the critical angle Angle of incidence GREATER than critical angle

Total internal reflection can turn a prism into a mirror!

The special property of TOTAL INTERNAL REFLECTION is used in OPTICAL FIBRES Uses of Optical Fibres include: Communication Endoscopes (the Magic Eye) Return to Menu

WEB LINK: http://sol.sci.uop.edu/~jfalward/physics17/chapter12/chapter12.html

Optical versus electrical Research why communications systems now use optical fibres instead of copper wires, use the table below to help you. Optical Electrical Information Attenuation Interference Cost Carry more Carry less Less More No Yes More Less How is attenuation solved in optical and electrical fibres? Regenerators for electrical cables Repeaters for optical fibres

Analogue: An Analogue signal can have any value. Think of a volume control or a dimmer switch. Singing into a microphone produces an analogue signal.

Analogue devices

Digital: An Digital signal can only have 2 values: 0 = No signal (off) 1 = Signal (on) Information is sent as pulses, often as radio waves (mobile phones) or laser light (fibre optic cables). The 0 and 1 information is called a Binary signal. The pulses can be used to send text, pictures, sound and movies. The shorter these pulses last for, the quicker information can be sent.

digital devices

Digital and analogue What is the difference between a digital signal and an analogue signal? What could they look like graphically? Digital signals can only be in one of two states, e.g. 0 or 1. Analogue signals are a continuously changing variable.

Analogue VS Digital Advantages of Analogue: Cheap, Easy (not much technology needed) Disadvantages of Analogue: Noise & loss of signal Quality is poor Cannot be copied more than a few times

Analogue VS Digital Advantages of Digital: Unlimited copies can be made Perfect quality No noise or interference Data can be copies to other applications Many signals can be sent at the same time (multiplexing) Signals can be compressed so they take less space to store Disadvantages of Digital: Requires computer technology More expensive

Digital compared to Analogue ADVANTAGES  Signals are clearer less susceptible to noise.  Can be used quickly by computers.  Carry digital signals using electromagnetic waves which travel at the speed of light.  Carry much more information. Digital hardware is much smaller. Easier to send over long distances. DISADVANTAGES  Digital hardware is expensive at the moment.  Although digital signals are unaffected by electrical interference, they don’t give a complete signal [just lots of samples] - some people feel that analogue vinyl records sound better than digital CDs for this reason. Electrical storms and random thermal noise. Noise is any unwanted information. Which signal is most prone to noise? Which signal carries the most information? Analogue Examples of noise are? What is noise? Digital

Communication systems Communication systems include the same basic components. You need to be able to define/describe the following: Modulator Transducer Encoder Transmitter Amplifier Storage Decoder Receiver

Definitions Component Function Encoder Decoder Modulator Transmitter Receiver Storage Transducer Amplifier Changes information into readable form Changes information to original form Allows wave to carry impulses AM/FM Makes oscillations Collects information Stores information e.g. CD, DVD, tape Changes information into electrical form or the other way round (microphone, speaker) Increases intensity of received waves

AM and FM AM FM What do AM and FM stand for? How do they compare? AM – amplitude modulation, where the sound wave is communicated by altering (modulating) the AMPLITUDE of the radio wave FM – frequency modulation, where the sound wave is communicated by altering (modulating) the FREQUENCY of the radio wave How do they compare? More noise AM FM Easier to send Less noise More noise Less noise Travels further Travels shorter distances Harder to send Travels further Travels shorter distances Easier to send Harder to send

Storage and retrieval of information What methods of storing information do you know? 1. _________________ 2. _________________ 3. _________________ 4. _________________ Magnetic Tape Analogue Digital Video Disc - DVD Digital Vinyl Records Analogue Compact Disc - CD Digital Which are digital and which are analogue?

COMMUNICATIONS: How do we send a phone or text message on our mobile to someone in Australia??? No worries, Mate..!

Radio waves How do radio signals from the UK reach around the globe? (1) The longer wavelength radio waves from a transmitter reflect off the Earth’s outer atmosphere (ionosphere).

How do radio signals from the UK reach around the globe? (2) Some microwaves can reflect off satellites and bounce around the planet.

Radio waves How can houses in the shadows of hills receive a radio signal? All electromagnetic radiation travels in straight lines, does it not? (3) Long wavelength radio waves can diffract around the hill to the house. Remember diffraction?

Which of the following is not a use of total internal reflection? Periscopes Endoscopes Fibre optic communication Mirrors 

What does attenuation mean? Strengthening Increasing Larger Weakening 

Which of the following is not a a digital storage method? DVD CD CD-ROM Vinyl record 