Topic 2

Topic 2

Beyond The Visible
1. William Herschel was a British Astronomer. He put dark coloured filters on his telescopes so he could observe the Sun safely.
  1. Noticed that different coloured filters heated up his telescope to different extents, wondered whether different amounts of light contained different amounts of heat.
    1. To test this, he : Used a prism to separate sunlight into a spectrum. Put a thermometer in one of the colours. Placed two other thermometers either side of the spectrum.
    2. Herschel wondered what would happen if he measured the temperature just beyond the red end of the spectrum, where there was no visible light
      1. This gave him the highest temperature. He had discovered infrared waves.
  2. Found out about Herschel's work. Then tried to find invisible rays at the opposite ends of the spectrum.
    1. Used silver chloride, a chemical which breaks down to give a black colour when exposed to light.
      1. He showed that silver chloride turned black fastest when exposed to invisible rays.
        These rays were later called ultra violet waves.
        Types of electromagnetic radiation : Visible light, Infrared and ultraviolet.
        These waves transfer energy from one place to another.
        Electromagnetic vibrations are at right angles to the direction in which energy is being transferred by the wave so they are transverse waves.
The Electromagnetic Spectrum and Electromagnetic Dangers
1. Electromagnetic waves can travel without any particles to vibrate.
  1. Meaning they can move easily through a vacuum, such as space.
    1. Electromagnetic waves travel at 300,000 kilometres per second in a vacuum.
      1. Colour of visible light depends on its wavelength.
        If the wavelength of a light wave is longer than that of red light, human eyes cannot see it.
        Waves such as Infrared, microwaves and radio waves have longer wavelengths than red light.
        Electromagnetic waves with shorter wavelengths have higher frequencies.
        Ultraviolet radiation has a higher frequency than visible light. Even shorter wavelengths are present in X rays and then gamma rays.
        Electromagnetic Spectrum - The full range of electromagnetic waves.
2. All waves transfer energy.
  1. A certain microwave frequency can heat water and this is used in microwave ovens.
    1. This can be dangerous to humans as we are mostly water. So mobile phones have different frequencies.
      1. IR radiation is used in grills and toasters to cook food.
        Our skin absorbs IR, which is felt as heat.
        Too much IR radiation can damage and destroy cells, causing burns to skin.
        Higher frequency waves transfer more energy an so are potentially more dangerous.
        Sunlight contains UV radiation which carries more energy than visible radiation
        Energy transferred by UV to our cells can damage their DNA.
        Too much exposure to UV can damage skin cells so much that it leads to skin cancer.
        Excessive exposure to X-rays and gamma rays may cause mutations or changes in DNA that may kill cells or cause cancer.
Electromagnetic and Ionising Radiation
1. The EURion pattern on banknotes can be seen when illuminated because it reflects certain wavelengths of visible light.
  1. These wavelengths can be detected by our wyes, photographic film, digital cameras, photocopiers and scanners.
    1. Some materials absorb UV radiation and re emits it as visible light. This is called fluorescence.
      1. It is used for security markings on property and banknotes, which can be checked with a UV lamp.
        Some security lights use fluorescent lamps.
        All objects give off some heat as IR radiation. CCTV cameras that detect IR are used to watch people at night.
        IR radiation can pass through fog, making thermal imaging useful in daytime too.
        IR signals are also sent down optical fibre cables for telephone and Internet communications.
        Gamma rays are used to kill cancer cells in radiotherapy.
        They are also used to detect cancer.
2. Gamma rays cause an increased risk of cancer by causing mutations in a cell's DNA.
  1. This is because gamma rays are ionising radiation.
    1. Such radiation can remove electrons from atoms to form ions.
      1. Some elements such as Radium give out gamma (y) waves all the time.
        Such elements are said to be radioactive.
        Others emit particles, called alpha (a) and beta (b) particles.
        All three are types of ionising radiation and transfer energy from the radioactive material to their surroundings.
        Alpha and beta particles are no electromagnetic radiation.
        They are particles of matter that are emitted with a lot of kinetic energy.
        This energy can ionise atoms.
        So like gamma rays they can be hazardous to life as they can also damage cells and DNA within the cells.

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