Nearly here Nailed i! Uni 1 Conen Opical fibres Opical fibres made from high-densiy glass can carry ligh signals long disances wihou losing any ligh hrough heir sides. Criical angle The criical angle, C, is he leas angle of incidence a which oal inernal reflecion dense medium occurs. Only applies when ligh ries o leave an opically dense (ha is, high refracive index, n) medium a a boundary ino a less dense (lower refracive index) medium. As angle of incidence increases, angle of refracion also increases and is always larger; some ligh is always refleced inernally. For incidence a he criical angle, C, he refraced beam would be a 90 ha is, along he boundary surface so i disappears and insead here is oal inernal reflecion. Ligh enering glass: c sin i n = = v sin r Ligh leaving glass: i and r are reversed. A criical angle, i = C and sin r = sin 90o = 1 so: sin C = 1/n C C Ligh pahs in a fibre cladding core cladding Ligh from a wide range of angles is refraced on enering he cu end of a fibre so ha i his is side surfaces a angles greaer han he criical angle, C, and so is oally inernally refleced. Some equaions will be given o you in he exam. Check he laes SAMs on he Pearson websie o see which equaions appear on he formula shee. Opical fibres versus copper wires Opical fibres are beer han copper wires because opical fibre signals: have lower losses, so ravel furher before needing amplificaion are secure, so canno be apped ino are a higher frequencies, so provide greaer bandwidh. u opical fibres cos more and need specialis insallaion. roadband neworks andwidh is a measure of he number of disinc signals a differen frequencies ha a nework can carry. High-frequency carrier waves can accommodae many Glass/air boundary air small bands of frequencies. Fibre opic cables allow very high bandwidh indeed, and so neworks using hem are called broadband. The long disance backbone of he broadband nework uses single mode fibres in which he dense glass core is so narrow (~8 μm) ha here is only one ligh pah, sraigh down he cenre of he fibre core. Very dense glass core: 8 µm diameer Jacke: 400 µm diameer uffer: Less dense 250 µm diameer glass cladding: 125 µm diameer Make-up of a single-mode opical fibre 1 Calculae he criical angle for ligh leaving glass of refracive index 1.40 o ener direcly ino air. 2 Give one example where you would choose o use copper cables o ransmi signals, and anoher example where opical fibres would be preferred. Explain your choices. 37
Uni 1 Conen Physics Nearly here Nailed i! Endoscopy undles of opical fibres make i possible o send ligh o and receive i back from places ha would oherwise be inaccessible for insance, inside he human body. Illuminaion A small fibre bundle is used for piping ligh from an exernal source down o he disal (remoe) end, where i illuminaes he area being invesigaed. Endoscope in use Along wih he supply of ligh, waer or air may be piped in and conrol wires for surgical insrumens can be included oo. Forming an image Lenses are used a boh ends: A iny objecive lens a he disal end of an endoscope focuses an image ono cu ends of he opical fibres in he main bundle. Each fibre in he bundle collecs and carries he ligh for one pixel (coloured do) in he image. An image obained from a fibrescope. The An eyepiece lens a he operaor end akes ligh exiing from he fibre bundle pixelaion of he image and refocuses i o be viewed as an image can be seen i is direcly by eye or wih a camera. made up of many small coloured dos. Analogue images Even hough divided ino pixels, hese images are analogue ha is, he brighness and colour of each pixel vary wih ime in direc proporion o he ligh colleced by he objecive lens. cannula Links arhroscopic insrumen arhroscope (camera and ligh source) For more on how analogue and digial images differ, see page 39. Key-hole surgery on a knee join The arhroscope shown in he box above is a small fibre opic camera endoscope designed for use inside joins. Compare his endoscope wih a microscope, highlighing he roles played by opical fibres. 38
Nearly here Uni 1 Nailed i! Conen Analogue or digial? A signal may eiher mimic he variaion in size and qualiy of he physical quaniy i represens an analogue signal or i can encode ha informaion as a series of numbers a digial signal. Meer displays Digial accuracy Sampling rae is he number of imes per second ha he quaniy is measured. Sampling sensiiviy is he smalles incremen in he quaniy ha is measured and recorded. These se a limi on he accuracy of digiised informaion. In a moving coil analogue meer, he needle moves in proporion o he curren flow in i ha he volage causes. A digial volmeer samples he volage a regular inervals and displays he resul each ime as a number. Analogue o digial converer (ADC) Images To se up: Connec a sensor (ransducer) o give an analogue signal ino he ADC inpu. Selec sampling rae and sensiiviy. All images, wheher prined, deeced by he rods and cones in he reina of your eye, or displayed on a TV screen, are made up of coloured dos, grains, pixels, ec. However, each do sars ou as an analogue signal. Send he digial oupu signal o a display or a daa sorage device. One of he do echniques used in prining 1 analogue in 0 1 0 1 0 1 digial ou ADCs are sandard elecronic componens. Digiised video convers each pixel ino a se of numbers represening is colour and brighness. Why ransmi in digial? Advanages of digial Mos daa is being swiched o digial ransmission, for example, radio, TV and all inerne raffic. Analogue sounds and picures are recreaed in he receiver. More daa fied in he same bandwidh. Mos inerference can be eliminaed giving higher qualiy. Can be sored and processed by a compuer. Drawbacks Time delay due o signal processing. Daa accuracy limied by sampling seings. An experimen requires emperaure o be moniored over a 24-hour period. A hermisor makes a useful small emperaure sensor. When conneced in a suiable elecronic circui, i will produce a volage signal ha varies direcly wih emperaure and can be displayed on a meer. 1 Describe, using a diagram, how you could adap his se-up o provide digial emperaure daa. 2 Explain wha advanages his could have over he simple analogue oupu o a volmeer dial reading. 39
Uni 1 Conen Physics Nearly here Nailed i! Elecromagneic waves All elecromagneic waves have he same speed, c, bu because hey have widely differing wavelenghs, frequencies and phoon energies you deec and use hem differenly. Inensiy he inverse square law Waves ha radiae in all Dish anennas direcions from an anenna have spherical wave-frons. So hey lose inensiy,, wih radius from he source, r, according o he equaion: = k/r 2 where k is he inensiy 1 m from he cenre of he source. Double he disance a quarer he inensiy Saellie communicaions use dish anennas ha concenrae he waves ino a direcional beam wih fla plane wavefrons. So hose waves lose inensiy more slowly. Some equaions will be given o you in he exam. Check he laes SAMS on he Pearson websie o see which equaions appear on he formula shee. The elecromagneic specrum Visible ligh ha is, he colours from deep red (λ = 740 nm) o indigo blue (λ = 370 nm) is he iny par of he elecromagneic specrum ha your eyes can deec. 400 500 600 700 visible ligh wavelengh (nanomeres) frequency, ff (Hz) 1021 1024 Gamma 10 14 1018 1015 Ulraviole X- 10 12 1012 10 9 Infrared 10 6 Microwave 10 3 106 109 103 AM radio wavelengh UHF VHF (TV) (FM) shor - medium - long wave 10 6 1 103 wavelengh (meres) Wha o memorise You only need o memorise eiher he wavelenghs or he frequency range for each par of he specrum. Then use c = f λ o calculae he oher (c = 3 108 m s 1). Infrared and longer wavelenghs are used for various communicaions purposes. Long waves penerae beer. High frequencies/ shorer waves carry more daa. Amospheric opaciy (%) The char shows which secions of he elecromagneic specrum are blocked by he amosphere. 1 Idenify he dangerous ionising radiaions ha are srongly absorbed, making life on Earh possible. 2 Explain which frequencies 100 are used for saellie communicaions. 50 3 Wavelenghs above abou 30 m are refleced by a layer of 0 charged paricles in he upper 0.01 nm 0.1 nm 1 nm 10 nm 100 nm 1 μm 10 μm 100 μm 1 mm 10 mm 100 mm 1 m amosphere. Commen on he Wavelengh impac of his for broadcasing. Char showing amospheric opaciy 40 10 m 100 m 1 km
Nearly here Nailed i! Uni 1 Conen Waves in communicaions You need o know he similariies and differences among saellie communicaions, mobile phones, Wi-Fi, lueooh and infrared daa ransfer. Geographical range eaming signals up and down beween saellies and ground saions allows hem o be sen righ round he world. Your mobile phone alks wih he ransceiver ower in a cell of a few km in radius. Each Wi-Fi hub has a range of 10 o 100 m. lueooh is limied o abou 10 m. Infrared signals reach only a few meres. Changing frequency To avoid inerference: Saellies have wo dish anennas and use quie differen frequency bands for uplink and down link. A ransponder uni on board receives, filers, amplifies and reransmis on he new frequency. Mobile phone ransceivers (ransmier-receivers) also use separae frequencies for uplink and downlink, bu hese are nearby channels. In addiion, each cell also uses a differen frequency from all is adjacen cells. lueooh devices frequency-hop across a range of channels many imes a second o limi heir inerference wih Wi-Fi, which operaes in he same frequency band. Neworking Wi-Fi hubs connec o a wired local nework and ogeher can cover a whole building. Mobile phone cells are neworked ogeher o cover he counry and also joined o he wider elephone nework. Similarly, saellie ground saions are linked ino communicaions neworks. Neworks use a combinaion of cable and wireless (for example, beamed microwave) links. lueooh and infrared are no neworked jus for device-o-device links. Handshaking A series of shor messages sen backwards and forwards allows one device o recognise anoher and o se up communicaion parameers. This is called handshaking. Handshaking also sops informaion geing los or jumbled by acknowledging when a message has been received. Mobile phones swap from one cell ower o anoher as a user ravels. 1 lueooh and Wi-Fi boh operae in he unlicensed 2.4 o 2.4835 GHz Indusrial, Scienific, Medical (ISM) band. Compare heir operaion and use for daa communicaions, and explain how hey co-exis wihou inerference. 2 Mobile phones (800 MHz o 2.6 GHz) and saellie communicaions (1 o 40 Ghz) boh operae in he same wo regions of he specrum. (a) Name hose regions. (b) Compare how each of hese echnologies makes use of he frequencies allocaed by he licensing auhoriies, and describe wha equipmen is involved. 41