Topic 2

Topic 2

1. Beyond The Visible

   • William Herschel was a British Astronomer. He put dark coloured filters on his telescopes so he could observe the Sun safely.
     • Noticed that different coloured filters heated up his telescope to different extents, wondered whether different amounts of light contained different amounts of heat.
       • 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.
       • Herschel wondered what would happen if he measured the temperature just beyond the red end of the spectrum, where there was no visible light
         • This gave him the highest temperature. He had discovered infrared waves.
     • Found out about Herschel's work. Then tried to find invisible rays at the opposite ends of the spectrum.
       • Used silver chloride, a chemical which breaks down to give a black colour when exposed to light.
         • 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.

2. The Electromagnetic Spectrum and Electromagnetic Dangers

   • Electromagnetic waves can travel without any particles to vibrate.
     • Meaning they can move easily through a vacuum, such as space.
       • Electromagnetic waves travel at 300,000 kilometres per second in a vacuum.
         • 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.
   • All waves transfer energy.
     • A certain microwave frequency can heat water and this is used in microwave ovens.
       • This can be dangerous to humans as we are mostly water. So mobile phones have different frequencies.
         • 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.

3. Electromagnetic and Ionising Radiation

   • The EURion pattern on banknotes can be seen when illuminated because it reflects certain wavelengths of visible light.
     • These wavelengths can be detected by our wyes, photographic film, digital cameras, photocopiers and scanners.
       • Some materials absorb UV radiation and re emits it as visible light. This is called fluorescence.
         • 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.
   • Gamma rays cause an increased risk of cancer by causing mutations in a cell's DNA.
     • This is because gamma rays are ionising radiation.
       • Such radiation can remove electrons from atoms to form ions.
         • 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.