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Noadswood Science, 2011 Saturday, April 2, 2016 EM & Communications To understand how EM waves are used in communications Anagrams Solve the following anagrams: Leetcmroanegitc murtsepc Drraenif Eeavwlnhgt Biisvle iltgh Magma Sary Qerenufcy Sary-X Leetcmroanegitc vwea Pamtildeu Zerth Anagrams Solve the following anagrams: Leetcmroanegitc murtsepc Drraenif Eeavwlnhgt Biisvle iltgh Magma Sary Qerenufcy Sary-X Leetcmroanegitc vwea Pamtildeu Zerth Electromagnetic spectrum Infrared Wavelength Visible light Gamma rays Frequency X-rays Electromagnetic wave Amplitude Hertz EM Spectrum Each type of radiation shown in the EM spectrum has a different wavelength, a different frequency and different uses Visible light, microwaves and radio waves are the main EM spectrum wavelengths used for communications… Radio Waves Radio waves are used for communications – there are 3 types: Short wave (3’000 – 30’000 kHz) only travel short distances but are very clear, e.g. FM Medium wave (300 kHz – 3 MHz) travel further than short wave, but are not as clear Long wave (>3 MHz) travel a very long way but are less defined Optical Fibres (Visible Light) Optical fibres can carry information coded in light or infrared signals – they can carry more information than an ordinary cable of the same diameter Information such as computer data and telephone calls can be converted into electrical signals – this information can also be converted into either visible light signals or infrared signals, and transmitted by optical fibres Optical fibres can carry more information than an ordinary cable of the same thickness – the signals in optical fibres do not weaken as much over long distances as the signals in ordinary cables Optical Fibres An optical fibre is a thin rod of high-quality glass (very little light is absorbed by the glass) Light getting in at one end undergoes repeated total internal reflection, even when the fibre is bent, and emerges at the other end Optical Fibres Light getting in at one end undergoes repeated total internal reflection, even when the fibre is bent, and emerges at the other end The critical angle for glass is about 42° Diamond’s have a critical angle of 24° – this is why they sparkle so much, due to many internal reflections Total Internal Reflection Total internal reflection occurs when light is coming out of something dense, such as glass, water or perspex If the angle is shallow enough the ray will not come out at all, but it reflects back into the material – this is total internal reflection Total Internal Reflection Angle of incidence is less than critical angle Most of the light passes through the air, but a little bit of it is internally reflected Total Internal Reflection Angle of incidence = critical angle The emerging ray comes out along the surface, with quite a lot of internal reflection Total Internal Reflection Angle of incidence is greater than the critical angle No light comes out as it is all internally reflected (total internal reflection) Optical Fibres - Uses Optical communications have several advantages over electrical signals in wires: The signal does not need boosting as often A cable of the same diameter can carry much more information The signals cannot be tapped into, or suffer interference from electrical sources Normally no light would be lost at each reflection, however some light is lost due to imperfections in the surface, so boosters are needed every few km Endoscopes Endoscopes are used to look inside people – they are a narrow bunch of optical fibres with a lens system at each end (with another set of optical fibres to carry light down inside) The image is displayed on a monitor, meaning operations can be undertaken without the need to cut large holes in the patient Phones How does a phone work – does it send sound up into space?! Watch the demo of the mobile phone in a vacuum Microwaves Microwaves are used by satellites because they can pass through the Earth’s atmosphere – they are used for mobile phone networks and satellite TV