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© Boardworks Ltd 2003 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.
© Boardworks Ltd 2003 Reflected ray Total Internal Reflection Incident ray Refracted ray Angle i Angle r At what angle of incidence did the ray change from refraction to reflection? It depends upon the material you used.
© Boardworks Ltd 2003 Simulation of a ray of light passing through a semi-circular glass block
© Boardworks Ltd 2003 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.
© Boardworks Ltd 2003 The Critical Angle How does refractive index affect the critical angle? The greater the refractive index the smaller the critical angle. MaterialRefractive index Critical angle Glass Water Diamond 42° 49° 24° Research the missing values below and then make a conclusion…
© Boardworks Ltd 2003 Total Internal Reflection Optical fibres, that are used in communication, use total internal reflection. 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. What are the applications of total internal reflection? You could be asked to draw on the path of the beam in an exam.
Angle of incidence LESS than critical Angle of incidence the SAME as critical Angle of incidence GREATER than critical results in TOTAL INTERNAL REFLECTION
Angle of incidence GREATER than critical angle Angle of incidence LESS than the 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
© Boardworks Ltd 2003 WEB LINK: hapter12.html
© Boardworks Ltd 2003 Optical versus electrical Research why communications systems now use optical fibres instead of copper wires, use the table below to help you. OpticalElectrical Information Attenuation Interference Cost Carry moreCarry less LessMore NoYes MoreLess How is attenuation solved in optical and electrical fibres? Regenerators for electrical cables Repeaters for optical fibres
© Boardworks Ltd 2003 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.
© Boardworks Ltd 2003 Analogue devices
© Boardworks Ltd 2003 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.
© Boardworks Ltd 2003 digital devices
© Boardworks Ltd 2003 Digital and analogue What is the difference between a digital signal and an analogue signal? Digital signals can only be in one of two states, e.g. 0 or 1. Analogue signals are a continuously changing variable. What could they look like graphically?
© Boardworks Ltd 2003 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
© Boardworks Ltd 2003 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
© Boardworks Ltd 2003 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. What is noise? Noise is any unwanted information. Examples of noise are? Electrical storms and random thermal noise. Which signal is most prone to noise? Analogue Which signal carries the most information? Digital Digital compared to Analogue
© Boardworks Ltd 2003 Communication systems Communication systems include the same basic components. You need to be able to define/describe the following: Encoder Amplifier Decoder Modulator Transmitter Receiver Transducer Storage
© Boardworks Ltd 2003 Definitions ComponentFunction Encoder Decoder Modulator Transmitter Receiver Storage Transducer Amplifier Changes information into readable form Allows wave to carry impulses AM/FM Changes information to original form Stores information e.g. CD, DVD, tape Makes oscillations Collects information Changes information into electrical form or the other way round (microphone, speaker) Increases intensity of received waves
© Boardworks Ltd 2003 AM and FM What do AM and FM stand for? 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? AMFM More noise Less noise More noiseLess noise Travels further Travels shorter distances Easier to send Harder to send
© Boardworks Ltd 2003 Storage and retrieval of information What methods of storing information do you know? 1._________________ 2._________________ 3._________________ 4._________________ Which are digital and which are analogue? Compact Disc - CD Digital Video Disc - DVD Magnetic Tape Vinyl Records Digital Analogue
© Boardworks Ltd 2003 COMMUNICATIONS: How do we send a phone or text message on our mobile to someone in Australia??? No worries, Mate..!
© Boardworks Ltd 2003 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).
© Boardworks Ltd 2003 (2) Some microwaves can reflect off satellites and bounce around the planet. How do radio signals from the UK reach around the globe?
© Boardworks Ltd 2003 How can houses in the shadows of hills receive a radio signal? All electromagnetic radiation travels in straight lines, does it not? Remember diffraction?(3) Long wavelength radio waves can diffract around the hill to the house. Radio waves
© Boardworks Ltd 2003 Which of the following is not a use of total internal reflection? A.Periscopes B.Endoscopes C.Fibre optic communication D.Mirrors
© Boardworks Ltd 2003 What does attenuation mean? A.Strengthening B.Increasing C.Larger D.Weakening
© Boardworks Ltd 2003 Which of the following is not a a digital storage method? A.DVD B.CD C.CD-ROM D.Vinyl record
© Boardworks Ltd 2003 Waves: Total Internal Reflection and Communication Give the meaning of critical angle Describe internal and total internal reflection.
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© Boardworks Ltd 2003 KS4 Waves : Dispersion and Electromagnetic radiation.
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