ELECTROMAGNETIC WAVES

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ELECTROMAGNETIC WAVES

• SECONDARY 3 PHYSICS

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WHAT ARE EM WAVES?

• Electromagnetic waves (EM waves for short) are
  waves that can travel in a vacuum.
• These waves are created by the vibration of an
  electric charge.

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• EM radiation is a wave that is produced as
  follows
• When charges accelerate, they produce a changing
  magnetic field.
• This changing magnetic field creates a changing
  electric field at 90 to it.
• The electric field now causes a changing magnetic
  field at right angles to it.
• The magnetic and electric fields are able to
  generate each other without any decrease in
  strength if the fields move at 3 ? 108 m.s-1.

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Electromagnetic Spectrum

• It is a group of different electromagnetic waves.
  
• There are 7 components in the spectrum.

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Electromagnetic Spectrum

• Gamma rays
• X rays
• Ultraviolet
• Visible light
• Infra-red
• Microwaves
• Radio waves

Highest frequency Shortest wavelength
Lowest frequency Longest wavelength
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Uses of Gamma Rays

• Kill cancer cells
• Study the nucleus in atoms

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Uses of X-rays

• Take pictures of bones in the body
• Study the crystal structure of crystalline
  substances
• Check for cracks in metal plates

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Uses of Ultraviolet

• Detect counterfeit notes
• Gives the clothes a glow effect in discotheques
• Sun-tanning
• Sterilise medical equipment

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Uses of Visible Light

• Enable us to see things
• Photosynthesis in plants

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Uses of Infra-red

• Heating
• Haze photography

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Uses of Microwaves

• Radar communication
• Analysis of the molecular and atomic structure
• Telephone communications

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Uses of Radio Waves

• Radar communications
• TV and radio broadcasting

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Common Properties of EM Waves

• All transverse waves
• All travel at the speed of light. (3 x 108 m/s)
• Can travel through solid, liquid, gas and vacuum
• Obey the laws of reflection and refraction
• All can be absorbed and emitted by matter
• The wave equation is applicable to all

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Penetrating ability of electromagnetic radiation

• The ability of EM radiation to go through
  (penetrate) bone, glass or concrete depends on
  the energy of the radiation and then also on the
  frequency of the radiation.
• Radio waves have the lowest frequency of EM
  radiation and thereby the lowest energy. They are
  not able to travel through the ground, therefore
  we cannot receive a radio signal underground. The
  radio waves are strongly diffracted (bent) around
  objects, while the shorter radio waves are
  reflected by the charged upper atmosphere
  fluctuate because of the changing reflecting
  ability of this layer.
•  

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• Microwaves have a low penetrative ability. Cell
  phone masts that use microwaves must have no
  obstructions in between them.
• Infrared radiation has low energy. It is able to
  warm the skin but cannot penetrate through the
  skin.
• Ultraviolet radiation has a frequency higher than
  violet light in the visible spectrum. The energy
  is such that it is harmful to the eyes and can
  cause the skin to tan.

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• X-rays have a high energy and a high penetrative
  ability. They can penetrate the soft tissue in
  the body but cannot go through the bone.
• Gamma rays can pass through a few centimetres of
  lead or concrete. They have a large amount of
  energy

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Energy carried by electromagnetic radiation

• Energy ? frequency
•  We calculate the energy of the radiation using
  
•  E h ? f
•  Where E is the energy of the radiation measured
  in Joules (J)
• h is plancks constant and has a value of
• 6,63 ? 10-34 J?s
• f is the frequency of the radiation measured in
  hertz (Hz)
•  

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• Since f c/ ?
•  To calculate the energy of the photon, we use
  the following equation
• E hc/ ?
• Where E Energy in joules (J)
• h planks constant (6,6 ? 10-34 J.s)
• f frequency in hertz (Hz)
• ? wavelength in metres (m)
• c speed of light (3 ? 108 m.s-1)