P15: Electromagnetic Waves

• Electromagnetic waves are transverse waves.
• All types of electromagnetic wave travel at the same velocity (the speed of light) in air or a vacuum.
• Human eyes can only detect visible light.

  The wavelength ranges from about 35nm to 650nm.
• The wavelength of an electromagnetic wave affects how it is absorbed, transmitted, reflected or refracted by different substances. This affects its uses.
• Radio Waves

  Uses: television, radio, bluetooth

  Radio waves are low energy waves and, therefore, not harmful, making them ideal for radio transmission.
• Microwaves

  Uses: satellite communications, cooking food

  Microwaves travel in straight lines through the atmosphere. This makes them ideal for transmitting signals to satellites in orbit and transmitting them back to receivers.
• Infrared waves

  Uses: electrical heaters, cooking food, infrared cameras

  
  

  Electrical heaters, grills, toasters, etc. glow red hot as the electricity flows through them. This transmits infrared energy that is absorbed by the food and converted back into thermal energy.
• Visible light

  Uses: Fibre optic communications

  Visible light travels down optical fibres from one end to the other without being lost through the sides.
• Ultraviolet waves

  Uses: energy efficient light bulbs, security marking, sun beds

  In energy efficient light bulbs, UV waves are produced by the gas in the bulb when it is excited by the electric current. These UV waves are absorbed by the coating on the bulb, which fluoresces going off visible light.
• X-rays

  Uses: medical imaging and treatments

  X-rays are able to penetrate soft tissue but not bone. A photographic plate behind a person will show shadows where the bones are.
• Gamma rays

  Uses: sterilising food and medical equipment, treatment of tumours

  Gamma rays are the most energetic of all electromagnetic waves and can be used to destroy bacteria and tumours.
• Changes in atoms and the nuclei of atoms can result in EM waves being generated or absorbed over a wide frequency range:

  • Electrons moving between energy levels from heat or electrical excitation can generate waves.
  • Changes in the nucleus of an atom can generate waves. e.g. an unstable nucleus can generate gamma rays.
• UV waves, X-rays and gamma rays carry enough energy to have hazardous effects on the human body:

  • UV waves can cause the skin to age prematurely and increase the risk of skin cancer.
  • X-rays and gamma rays are ionising radiation - they can damage cells by ionising atoms. If absorbed by the nucleus of the cell, can cause gene mutations and cancer.
• The risk of damage from EM waves depends on the type of radiation and the amount of exposure.

  Radiation does is a measure of harm based on these two factors. It is measure in Sieverts (Sv).
• 1. Radio waves can be generated by an alternating current (oscillations in electrical circuits)
  2. The frequency of the radio waves produced match the frequency of the alternating current. This is how a signal is produced.
  3. The radio wave is then absorbed by a conductor, which will create another matching alternating current. This is how the signal is received(on a receiver).
• Microwaves can heat up internal parts of peoples bodies.

  Infrared can cause skin burns.
• White light contains all the colours of the visible spectrum.

  It reflects all colours back.
• X-rays are ionising radiation and so they can damage living tissue when they pass through it. As a result, you don't want to have too many X-rays in your lifetime.
• The shorter the wavelength the more energy a wave has.