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  1. 1. KS4: Electromagnetic Waves
  2. 2. Electromagnetic Spectrum  When particles [usually electrons] accelerate or decelerate, they make electromagnetic waves.  Electromagnetic waves are transverse waves made up of electric and magnetic fields which travel together.  All electromagnetic waves can travel through space.  All electromagnetic waves travel at the same speed [300,000,000 m/s in a vacuum].
  3. 3. Electromagnetic Spectrum  Visible light is made when negative electrons slow down as they move around inside an atom.
  4. 4. Electromagnetic Spectrum  Although all e-m waves travel at the same speed, their wavelength [λ] and frequency [ƒ] can change.  The properties, dangers and uses of e-m waves depends on the wavelength [λ]. Waves that cook food. Waves that cause sun-tans. Waves that cause cancer.
  5. 5. Electromagnetic Spectrum  The whole family of electromagnetic waves is called the electromagnetic spectrum. Radio Micro Infra-Red Visible Ultra-Violet Xrays Gamma λ increases ƒ increases
  6. 6. Electromagnetic Spectrum Name Gamma Rays λ 0.000 000 001 mm Properties  Very high energy  Pass through body unchanged  VERY dangerous Uses  Kill cancer cells [radiotherapy]  Sterilise medical equipment  As tracers to look at lung structure
  7. 7. Electromagnetic Spectrum Name X rays λ 0.000 001 mm Properties  Energetic  Short wave X rays pass through flesh but not bone  Dangerous Uses  Look through body i.e. broken bones and teeth  Scan luggage for dangerous items
  8. 8. Electromagnetic Spectrum
  9. 9. Electromagnetic Spectrum Name Ultra Violet λ 0.000 01 mm Properties  λ is too short for eyes to see  causes sun-tans Uses  ‘sun’ beds  checking counterfeit banknotes  in nightclubs The £5 note on the left is genuine. The note on the right glows in UV and is counterfeit
  10. 10. Electromagnetic Spectrum Name Visible Light λ 0.005 mm Properties  Our eyes respond to these λ  Made of ROYGBIV Uses  To see!  Expose photographic film  To generate electricity in photo- electric cells [solar panels]
  11. 11. Electromagnetic Spectrum Name Infra Red λ 0.01 mm Properties  Emitted by warm objects  Hotter the object, shorter λ emitted Uses  Remote controls  Thermal imaging cameras  Electric grill
  12. 12. Electromagnetic Spectrum Name Microwaves λ 1 mm – 1 cm Properties  Can carry information  Some λ absorbed by food Uses  Microwave ovens  Mobile communications  RADAR Microwaves reflect around the oven and carry energy to about 1 cm into the food. This cooks the food quickly
  13. 13. Electromagnetic Spectrum Name Radio λ 10 cm – 1 km Properties  Can carry information  Travel large distances Uses  Broadcasting [carry radio and TV signals] 96.7 FM Radio waves are emitted when electrons move up and down an aerial very quickly.
  14. 14. Electromagnetic Spectrum 1) Match up the following parts of the electromagnetic spectrum with their uses : Gamma rays Allow us to see Radio waves Remote Controls Ultra Violet ‘See’ broken bones Visible Carry TV signals Microwaves RADAR X rays Sterilise equipment Infra Red Causes sun-tans
  15. 15. Electromagnetic Spectrum 1) A radio station uses waves of frequency 96.7 MHz If the speed of e-m waves in air is 300,000,000 m/s, a) calculate the wavelength of the radio waves used. b) calculate the time taken for the transmission to travel 50 km. 2) Why can we see the Sun but can’t hear it? 3) Write down 3 things all e-m waves have in common.
  16. 16. Reflection : A reminder  From KS3 you should remember :  Pale and shiny surfaces are good reflectors, dark and rough surfaces are not.  The image in a plane mirror is laterally inverted.  The image is the same distance behind the mirror as the object is in front.  The image in a plane mirror is the same size as the object.  angle of incidence = angle of reflection ¡ = r
  17. 17. Reflection : A reminder Angle i Angle r ¡ = r Incident ray reflected ray
  18. 18. Reflection : Curved Mirrors  In KS3 you just dealt with plane mirrors.  By curving a mirror, we can make mirrors more useful: Concave mirrors curve inwards Convex mirrors bulge outwards
  19. 19. Reflection : Curved Mirrors ƒ Chose a distant object [to get parallel rays of light]. Finding ƒ of a concave mirror. Hold the mirror in the other hand and move it closer to the screen until a clear image appears. Hold a plain white screen in one hand. Use a ruler to measure the distance between the lens and the screen - this is the focal length [ƒ].
  20. 20. Reflection : How does curvature affect ƒ ?  Concave mirrors reflect rays of light to a focal point.  The distance between the mirror and the focal point is called the focal length [ƒ].  How can ƒ be changed?  Concave mirrors produce real images because the rays of light meet [unless the object is close].
  21. 21. Reflection : How does curvature affect ƒ ? ƒ  Take a piece of Al or stainless steel sheet and curve it slightly.  Shine parallel rays of light at the reflector and plot their positions.  Draw around the reflector.  Measure ƒ and record your results.  Carefully bend the reflector and repeat the process to see how ƒ changes with curvature.
  22. 22. Reflection : Convex mirrors ƒ  Convex mirrors reflect rays of light away from a focal point.  The distance between the mirror and the focal point is called the focal length [ƒ] Convex mirrors produce virtual images - the rays of light do not meet.
  23. 23. Reflection : Curved mirrors  Concave reflectors are used to focus signals from distant satellites.  Convex reflectors are used to widen the field of view.
  24. 24. Total Internal Reflection Incident ray Reflected ray Refracted ray Angle i Angle r Refraction or Reflection? 15° 30° 45° 60° 75° Angle i Angle r Angle r  At what angle of incidence did the ray change from refraction to reflection?
  25. 25. 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.
  26. 26. Total Internal Reflection i = r Optical fibre
  27. 27. Total Internal Reflection  Why do communications systems now use optical fibres instead of copper wires? ADVANTAGES  Can carry much more information as digital signals.  Carry information at the speed of light [300, 000 km/s].  Clear signals unaffected by electrical interference. DISADVANTAGES  Expensive to make as very high quality glass is needed.  Need careful handling - signal loss if cracked.
  28. 28. Refraction : A reminder  When light bends this is called refraction.  Refraction happens because the light changes speed [or velocity].  If the incident ray hits a surface at 0º, no refraction occurs. air glass
  29. 29. Refraction : Lenses  At KS4, you need to be able to explain how to change the size and nature of an image formed by a convex lens.
  30. 30. Refraction : Lenses 1. Find the focal length [ƒ] of your lens. 2. Fix the lens to the centre of a metre rule and mark the distances F and 2F either side of the lens. 2F F F 2F 3. Place the candle >2F away from the lens and move the screen until an image appears. 4. Measure the distances between the candle, image and lens and describe the image in the results table.
  31. 31. Refraction : Lenses Object position [as F] Distance from O to lens [cm] Image position [as F] Distance from I to lens [cm] Image Descrip tion Graph >2F away 2F away between F & 2F at F between F and lens Magnif ication
  32. 32. Refraction : Lenses  Object >2F away O 2F F F 2F I  The image [ l ] is formed between F and 2F away from the lens, is inverted and diminished.
  33. 33. Refraction : Lenses  Object at 2F O 2F F F 2F I  The image [ l ] is formed at 2F away from the lens, is inverted and the same size.
  34. 34. Refraction : Lenses  Object between 2Fand F away O 2F F F 2F I  The image [ l ] is formed further than 2F away from the lens, is inverted and magnified.
  35. 35. Refraction : Lenses  Object at F away O 2F F F 2F  The image [ l ] is formed at infinity - the rays never meet [we use this set-up for searchlights].
  36. 36. Refraction : Lenses  Object between F and lens O I  The VIRTUAL image [ l ] is formed on the same side of the lens as the object, is the right way up and magnified. 2F F F 2F
  37. 37. Refraction : Lenses Magnification = Distance from lens to image Distance from object to lens 2F F F 2F
  38. 38. Using Refraction : Sound  Sound waves can be refracted as well as light waves  Move the microphone across the balloon and watch the CRO trace of the sound wave.  What does the CO2 in the balloon do to the sound waves? Why? CO2
  39. 39. Diffraction & Interference  Waves travel in straight lines but when they go past an edge they spread out in a new direction.  This is called diffraction
  40. 40. Diffraction & Interference  When 2 waves meet, they interfere with each other.  If they meet each other exactly in phase, the amplitudes ‘add up’ to produce large crests and troughs. + =  This is called constructive interference.
  41. 41. Diffraction & Interference  If they meet each other exactly out of phase, the amplitudes ‘subtract’ to produce no peaks or crests. + =  This is called destructive interference.
  42. 42. Diffraction & Interference  To get 2 waves of light to interfere, the waves must be very similar.  We use a single source of monochromatic light and split it into 2 waves using a diffraction grating like this:  In 1801, a physicist called Young first performed this classic investigation which showed the interference of light waves.
  43. 43. Diffraction & Interference The light source emits rays of light which diffract towards the double slit S1 S2 S1 and S2 act as 2 coherent light sources  The waves interfere - constructively [bright fringes]. destructively [dark fringes]. Fringes
  44. 44. Diffraction & Interference  What would the fringes look like if white light was used as the source instead?
  45. 45. Diffraction & Interference  The coloured fringes on these CDs are the result of interference. Light reflecting from the Aluminium diffracts and interferes. Some colours are diffracted more than others.
  46. 46. Communication Communicate v. Make known; transmit; pass information to and fro; have means of access  To pass information quickly over large distances, we use waves. These dishes collect and focus microwaves from a communications satellite 100’s of km above the Earth.  The effects of reflection, refraction and diffraction are important to consider when designing communications systems.
  47. 47. Communication The meters at the top are analogue meters - they use a needle to represent the reading. The digital meters below give the reading as a number.  Computers handle digital readings much faster and easier. 3.4 2.6
  48. 48. Communication 0 1 2 3 4 5 6 0 1 2 3 4 5 Time [1/10,000 s] SamplingLevel  Use the chart on the next page to turn the analogue signal into binary code and then a voltage sequence.
  49. 49. Communication Sampling Level Binary Code Voltage Sequence 0 000 LOW LOW LOW 1 001 LOW LOW HIGH 2 010 LOW HIGH LOW 3 011 LOW HIGH HIGH 4 100 HIGH LOW LOW 5 101 HIGH LOW HIGH 6 110 HIGH HIGH LOW
  50. 50. Communication ADVANTAGES  Signals are clearer.  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. 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.
  51. 51. Communication ionosphere Transmitter dish Receiver dish Gugliemo Marconi first reflected radio waves off the ionosphere in 1901 [from England to Canada].
  52. 52. Communication Transmitter dish Receiver dish The UHF radio waves we use for TV carry a lot of information but don’t reflect off the ionosphere. We use communications satellites which amplify and transmit the signal.
  53. 53. Communication : Diffraction  UHF radio waves carry high quality TV signals but can’t diffract round hills very well - you get a poor signal in valleys.  LW and MW signals diffract round hills so you get a good signal in valleys.
  54. 54. Communication : Diffraction  Waves from the transmitter dish spread out due to diffraction.  The receiver dish can’t collect all the waves and so some energy is wasted - the signal must be amplified.