07/04/2010 Radiation and the Universe Electromagnetic Radiation 07/04/2010 E-M radiation is basically a movement of energy in the form of a wave. Some examples: The Electromagnetic Spectrum 07/04/2010 Each type of radiation shown in the electromagnetic spectrum has a different wavelength and a different frequency: High frequency, Low frequency, _____ _____ wavelength (high) wavelength Gamma X-rays Ultra violet Visible Infra red Microwaves Radio/TV rays light γ Each of these types travels at the same speed through a _______ (300,000,000m/s), and different wavelengths are absorbed by different surfaces (e.g. infra red is absorbed very well by ___________ surfaces). This absorption may heat the material up (like infra red and _______) or cause an alternating current (like in a __ _______). Words – black, microwaves, long, short, TV aerial, vacuum The Electromagnetic Spectrum 07/04/2010 Type of radiation Uses Dangers Gamma rays Treating cancer, Cell mutation sterilisation X rays Medical Cell mutation Ultra violet Sun beds Skin cancer Visible light Seeing things None (unless you look at the sun) Infra red Remote controls, Sunburn heat transfer Microwaves Satellites, phones Very few TV/radio Communications Very few Transmitting information 07/04/2010 Although E-M radiation travels in straight lines, we can send infra-red and light signals around a curved path using an optical fibre: Optical fibres have two main advantages: they can send more information compared to electrical cables of the same diameter and with less signal weakening. Microwaves are used by satellites because they can pass through the Earth’s atmosphere: Microwaves are also used in mobile phone networks. The Wave Equation 07/04/2010 All E-M waves obey the Wave Equation: Wave speed (v) = frequency (f) x wavelength () in m/s in Hz in m V f Analogue vs. Digital Signals 07/04/2010 Analogue signals (like talking or + music) continually vary in amplitude and/or frequency - 1 Digital signals, however, are either off or on, and the information is sent 0 in a series of pulses There are two main advantages of digital: 1) More information can be sent down the same cable 2) Better quality, because a digital signal can be amplified without amplifying the extra noise: The structure of the atom 07/04/2010 ELECTRON – negative, mass nearly nothing PROTON – positive, same NEUTRON – mass as neutral, same neutron (“1”) mass as proton (“1”) The structure of the atom 07/04/2010 Particle Relative Mass Relative Charge Proton 1 +1 Neutron 1 0 Electron 0 -1 MASS NUMBER = number of protons + number of neutrons SYMBOL PROTON NUMBER = number of protons (obviously) Isotopes 07/04/2010 An isotope is an atom with a different number of neutrons: Notice that the mass number is different. How many neutrons does each isotope have? Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more. A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating. Introduction to Radioactivity 07/04/2010 Some substances are classed as “radioactive” – this means that they are unstable and continuously give out radiation: Radiation The nucleus is more stable after emitting some radiation – this is called “radioactice decay”. Types of radiation 07/04/2010 1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a Unstable New Alpha ______ atom) nucleus nucleus particle 2) Beta () – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, Beta high energy electron is called a _____ particle particle. Unstable New nucleus nucleus 3) Gamma – after or decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed. Unstable New Gamma Words – frequency, proton, nucleus nucleus radiation energy, neutrons, helium, beta Ionisation 07/04/2010 Radiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by “knocking off” electrons: Alpha radiation is the most ionising (basically, because it’s the biggest). Ionisation causes cells in living tissue to mutate, usually causing cancer. Blocking Radiation 07/04/2010 Each type of radiation can be blocked by different materials: Sheet of Few mm of Few cm of paper aluminium lead Deflection by Magnetic Fields 07/04/2010 + 2 protons, 2 neutrons, Alpha and beta particles + therefore charge = +2 have a charge: 1 electron, therefore - charge = -1 Because of this charge, they will be deflected by electric and magnetic fields: + - Background Radiation 07/04/2010 13% are man-made Radon gas Food Cosmic rays Gamma rays Medical Nuclear power Uses of radioactivity 07/04/2010 1) Medical uses – gamma rays can be used to destroy cancerous cells or to sterilise medical instruments 2) Tracers – a tracer is a small amount of radioactive material used to detect things, e.g. a leak in a pipe: Gamma source The radiation from the radioactive source is picked up above the ground, enabling the leak in the pipe to be detected. Tracers can also be used to develop better plant fertilisers and in medicine to detect tumours: Uses of radioactivity 2 07/04/2010 Beta detector Rollers Paper Beta emitter Dangers of radioactivity 07/04/2010 Radiation will ionise atoms in living Alpha cells – this can damage them and cause cancer or leukaemia. Beta Gamma OUTSIDE the body and are more dangerous as radiation is blocked by the skin. INSIDE the body an source causes the most damage because it is the most ionising. Half life 07/04/2010 The decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay… = radioisotope = new atom formed After 1 half After 2 half After 3 half At start life half have lives another lives another there are 16 decayed half have 2 have radioisotopes (that’s 8) decayed (12 decayed (14 altogether) altogether) A radioactive decay graph 07/04/2010 Count Time 1 half life Dating materials using half-lives 07/04/2010 Question: Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample. Answer: The sample was originally completely uranium… 1 half life 1 half life 1 half life later… later… later… 8 4 2 1 8 8 8 8 …of the Now only 4/8 of Now only 2/8 of Now only 1/8 of sample was the uranium uranium remains uranium remains uranium remains – the – the other 6/8 – the other 7/8 other 4/8 is lead is lead is lead So it must have taken 3 half lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead). Each half life is 4,000,000,000 years so the sample is 12,000,000,000 years old. An exam question… 07/04/2010 Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium. How old is the rock? (3 marks) The rock must be 2 half lives old – 2.6 billion years 07/04/2010 Evidence about the origins of the universe… 07/04/2010 Source of light “Spectra” 07/04/2010 If you pass the light through a gas something different is seen… helium Some wavelengths of light are absorbed by the gas – an “absorption spectrum”. 07/04/2010 If the light source is moving away the absorption spectra look a little different… Before helium helium After 07/04/2010 The absorption lines have all been “shifted” towards the longer wavelength end (red end)… Before This is called red shift. The faster the light source moves the further its light will be “shifted” After A similar effect happens with sound – this is called “The Doppler Effect” Hear Doppler Effect 07/04/2010 Light from different stars and from the edge of the universe also shows this “red-shift”. This suggests that everything in the universe is moving away from a single point. This is the BIG BANG theory Red shift summary 07/04/2010 Light from other galaxies has a longer _________ than expected. This shows that these galaxies are moving ____ from us very quickly. This effect is seen to a greater extent in galaxies that are _______ away from us. This indicates that the further away the galaxy is, the ______ it is moving. This evidence seems to suggest that everything in the universe is moving away from a single point, and that this process started around 15 _____ years ago. This is the ____ ________ Theory. Words to use – faster, away, big bang, billion, wavelength, further Observing the Universe 07/04/2010 Consider different types of telescope: Ground-based telescopes Space-based telescopes What are the advantages and disadvantages of each?