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					The Electromagnetic Spectrum



        Year 11 Physics
 What is an Electromagnetic Wave?
• Electromagnetic waves are transverse waves.
• They consist of alternating electric and magnetic
  force fields at 90 degrees to one another and in the
  direction of energy transfer.
• These forces are generated by changes in the speed
  or direction of moving electric charge.
Some properties of Electromagnetic Waves
• All electromagnetic waves can pass through a vacuum.
• Electromagnetic waves travel the vacuum of space at the
  common speed of light of 300 million metres per second (3.0 x
  108 ms-1)
• Almost all of the energy that reaches the Earth from the Sun is
  in the form of electromagnetic radiation.
• It takes 8 minutes for light from the Sun to reach the Earth.
      What is the Electromagnetic Spectrum?
• The Electromagnetic Spectrum is a continuum of electromagnetic
  waves with artificial divisions based on the frequency and
  wavelengths of the waves.
• There is no distinct point at which the frequency changes and no
  special change in properties at particular waves boundaries (looking
  at a rainbow illustrates this).
• Simply oscillating electrons in a wire or aerial can
  produce low frequency electromagnetic waves,
  like radio and television waves.
• Light waves oscillate too rapidly in this way and
  are produced by the outer electrons changing
  energy levels (shells) in atoms.
• X-rays are produced when the inner electrons
  change energy levels.
• Gamma rays, which have extremely high
  frequencies, are produced by energy changes in
  the atomic nucleus.
                     Radio Waves
• Wavelengths ranging from 10cm to 1000m.
• Lowest energy waves in Electromagnetic Spectrum.
• Radio Waves include: AM radio, FM radio, TV, Microwaves and
  Radar.
• Detected by aerials connected to tuned electric circuits in radios
• Variety of uses – depends upon frequency (see below):
              AM and FM Radio
• In AM (Amplitude Modulation) the audio signal
  changes the amplitude of the carrier wave.
• In FM (Frequency Modulation) the audio signal
  changes the frequency of the carrier wave.




• AM radio waves have longer wavelengths than FM and
  can be received at greater distances.
• FM radio waves are less affected by electrical interference
  and hence provide a higher quality transmission of sound
                Television
• Television signals are transmitted on two
  separate carrier waves
  – Visual signal is added onto one carrier wave
    using Amplitude Modulation (AM)
  – Audio signal is carried on a separate carrier
    wave using Frequency Modulation (FM)
• When you select a particular channel, you
  are selecting the respective visual and audio
  carrier waves for that channel.
• Your TV then completes the task of
  ‘stripping’ the carrier waves to produce the
  desired picture and sound.
                   Microwaves
• Wavelengths ranging from 1 millimetre to 30
  centimetres.
• Were first used in World War 2 in Radar.
• Used in microwave ovens (frequency of 2450 MHz) for
  cooking. Produced by a magnetron when cathode rays
  (a beam of electrons) rotate past an electric field.
• Also, used in mobile phone communications at
  frequencies of around 900 MHz. Transmission can be
  across distances of up to 100 km, but there must be a
  direct ‘line of sight’
• Detected in the same way as radio waves and television
  signals
                    Infra-red Radiation
• Wavelengths ranging from 700 nanometres (0.0007 millimetre) to 1
  millimetre.
• Emitted by hot objects
• Detected by special photographic film and semiconductor devices
• Variety of uses including:
    –   Remote controls
    –   Security and burglar alarms
    –   Medical treatments for soft tissue injury.
    –   Thermal imaging applications.
                        Visible Light
• Wavelengths ranging from 400 to 700
  nanometres.
• We see light of different frequencies as
  different colours.
• White light is light that contains all the
  colours of the spectrum
• Detected by the eyes, photographic film
  and photo cells
• A variety of applications including:
   – fibre-optic communications
   – Photography
   – Laser technology
       Ultraviolet (UV) Radiation
• Wavelengths ranging from 10-400 nanometres.
• Small doses beneficial to humans as it encourages production of
  vitamin D.
• Larger doses can lead to cell and tissue damage – possibly causing skin
  cancer or eye cataracts.
• Most types of glass absorb UV rays but clouds do NOT absorb UV
  (that is why you can get sunburnt on cloudy days)
• Detected by photographic film, photo cells and fluorescent chemicals
• Variety of uses including:
   – Photo-initiator chemicals in polymerisation
   – Astronomical observations
   – Sterilisation of hospital equipment
                             X-rays
•   Wavelengths ranging from 0.01-10 nanometres.
•   Have energy enough to pass through human flesh
•   Detected by photographic film and fluorescent screen
•   Variety of uses including:
     – Cancer treatment by focussing the rays to kill cancer cells
     – Finding weakness in metals and analysing structures of
       complex chemicals.
     – Imaging applications in medicine.
X-ray Images
                 Gamma Rays
• Wavelengths less than 0.01 nanometres.
• Highest energy waves in Electromagnetic Spectrum.
• Produced when energy is lost from the nucleus of an atom
  during radioactive decay.
• Detected by photographic film or a Geiger-Müller counter.
• Highly destructive to human tissue.
• Can be used to kill cancer cells.
• Also used in finding fractures and weaknesses in metals.
        Atmospheric Filtering
• Only a small range of the frequencies in the
  electromagnetic spectrum reach the Earth’s
  surface
• The Earth’s atmosphere and ionosphere
  absorb the rest
• Very little ultraviolet, X-ray or gamma
  radiation penetrates the atmosphere (a good
  thing)
• The ionosphere is the upper layer of the
  atmosphere in which the gaseous atoms and
  molecules have become ionised (gained or lost
  electrons)
• The ionosphere itself can be divided into three
  layers: D, E and F
• D: 50 – 80 km above Earth’s surface, absorbs
  short wavelength (hard, high energy) X-rays
• E: 80 – 105 km above Earth’s surface, absorbs
  long wavelength (soft, low energy) X-rays
• F: 145 – 300 km above Earth’s surface, absorbs
  short wavelength UV-rays

				
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posted:10/3/2012
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