Radiation and the universe (summary of AQA module)

1. Radiation and the universe summary 14 February, 2011

2. The electromagnet spectrum Electromagnetic waves are electrical and magnetic disturbances that transfer energy from one place to another. Their wavelengths can vary from very long(radio waves) to very short (gamma waves). They travel at 300 000 000 metres per second. The order of the spectrum is: radio, microwaves, infra red, visible, ultra violet, X rays and gamma waves 1

3. Gamma rays and x rays Gamma rays and x rays are very short, high energy and highly penetrating waves. They can be absorbed by denser atoms such as calcium in bones and metals. X rays are used to take radiographs of the body in hospitals. Gamma rays are used to sterilize surgical equipment, kill bacteria in food and kill cancer cells. 2

4. Light and ultra violet waves White light consists of all the colours of the visible spectrum. The colours can be separated by dispersion in a prism. Ultra violet waves are shorter than visible waves and like light can be absorbed, reflected or transmitted through substances. Ultraviolet rays damage the human skin and eyes. 3

5. Infra red, microwaves and radio waves. Infra red waves are emitted from hot objects and are used in scanners, optical fibres and remote hand sets. Microwaves are used in heating food (ovens) and communications (mobile phones). Radio waves are used in communications – the longer waves diffract better so can be transmitted over longer distances. 4

6. Communications 5 Radio waves are extensively used for communications. Different frequencies have different uses including: radio, FM radio, TV, mobile phones and satellite links. Higher frequency waves carry more information but have a short range because they are absorbed by the atmosphere and don’t diffract (bend around corners)much. The ionosphere reflect the longer waves. Optical fibres carry information in the glass fibre as it reflects along the thin fibre.

7. Analogue signals Analogue signals are wave signals that vary continuously in amplitude and frequency. Example is a microphone. The signal is transmitted in a carrier wave from a transmitter to a receiver. Analogue waves are subject to interference so they often carry a lot of noise and distortion. 6

8. Digital signals A digital signal is a series of pulses. The voltage of the pulse is either high (1) or low (0). Each pulse is called a bit. Digital signals are also set by carrier waves to a receiver. However the receiver can clean the noise from the pulses so there is no interference. Digital signal carry more pulses per second than analogue. These are used in mobile phones, radio and computers. 7

9. Nuclear radiation Many of the larger, heavier atoms are unstable and release radiation to become more stable. The radiation can be detected by a Geiger counter and photographic film. Radioactive decay is a random event for a particular atom but occurs regularly when large numbers of atoms are present. The radiation released is dangerous as the high energy particles or waves can destroy or modify cells causing cancer. 8

10. Alpha particles Alpha particles are helium nuclei – they have a mass of 4 atomic mass units and a charge of +2. These particles are released from the nucleus of an unstable atom. They have a short range in air (about 10 cm) and are absorbed by paper. They are very dangerous inside living cells. 9

11. Beta particles Beta particles are electrons which have been emitted from the nucleus of an unstable atom. They have a very small mass and a charge of -1. These particles can penetrate paper but are absorbed by thin metal sheets. (1cm aluminium). They are dangerous from outside the living cell. 10

12. Gamma radiation Gamma rays are very short wave, high frequency electromagnetic radiation. They have no mass and no charge. They are highly penetrating and can only be stopped by thick lead sheet or very thick concrete. Their range in air is unlimited. They easily penetrate living cells from outside the body. 11

13. Half life of a radioactive substance Radioactive atoms decay at random, however when there are large number of the atoms present the average rate of decay can be predicted. The half life of a radioactive substance is the time it takes for half of the active unstable particles to decay. The number of radioactive particles decreases in the sequence 1: ½:1/4: 1/8 and so on for every subsequent half life. The level of radiation is measured on a Gieger counter. 12

14. Radioactivity at work Some uses of radioactive particles are: (1) Thickness monitoring – using a beta particle source attached to detector which applies varied pressure to rollers. (2) Radioactive tracers – short half life gamma sources are injected into a patient to track the flow of fluids in the body. (3) Radioactive dating – the amount of radioactive carbon or uranium in material can be used to track the age of past living tissue and rocks respectively. 13

15. The expanding universe The Doppler effect shows that objects in front of a moving source would see light waves shortened (blue shifted) while those behind the source see the waves lengthened ( red shifted). Light from distant galaxies is always red shifted which suggests that they are all moving away from our galaxy. This indicates that the universe is expanding and has been doing this for about 13 billion years. 14

16. The Big Bang The fact that the universe is expanding suggests that all matter was created at one point at one time (singularity) – this was the Big Bang. Since that time all matter in all the galaxies have been moving apart. At the start of the universe a huge amount of energy was released – this energy is still observable today in the form of background microwave radiation. It is still uncertain as to whether the universe will continue to expand or will eventually collapse under gravity. 15

17. Looking into space The Earth’s atmosphere absorbs all electromagnetic waves except light, radio waves and some ultra violet rays. For the above reason observing space from the Earth only give a partial view of the universe. Images for satellites like the Hubble telescope give a true detailed image of the electromagnetic radiation from all the universe. Telescopes and radio telescopes work from the Earth’s surface provided there is no interference for these waves. 16