Physics P1b

• The electromagnetic spectrum, (in order of increasing wavelength); gamma rays, x-rays, ultraviolet, visable, infar-red, microwaves, radiowaves
• All electromagnetic waves travel at the same speed in space (300million m/s)
• Wave speed= frequency x wavelength

• X-rays and gamma radiation are absorbed by dense materials in bone and by metal
• X-rays and gamma radiation damage living tissue when they pass through it
• X-rays are used in hospitals to radiographs
• Gamma rays are used to kill harmful bacteria in food, to sterilise surgical equipment and to kill cancer cells

• Ultraviolet radiation is in the electromagnetic spectrum between violet light and X-radiation
• Ultraviolet radiation has a shorter wavelength than viable light
• Ultraviolet radiation can harm the skin and the eyes

• Infra-red:Heaters, communication (remote handsets, optical fibres)
• Microwaves: Microwave oven, comminications
• Radio waves: Communications
• Communications- the use we make of the radio waves depend on the frequency of the waves, Visible light and infra-red radiation are used to carry signals in optical fibres

• Analogue signals vary contiously in amplitute
• Digital signals are either high (1) or low (0)
• Digital transmission, when compared with analogue transmission is free of noise and distortion. It can also carry much more information

• A radioative substance contains unstable nuclei
• An unstable nucleus becomes stable by emitting radiation
• There are three main types of radiation from radioactive substances-Alpha, Beta and gamma radiation
• Radioactive decay is a random event-we cannot predict or influence when it will happen

• Alpha radiation is stopped by paper or a few centimetres of air
• Beta radiation is stopped by thin metal or about a metre of air
• Gamma radiation is stopped by thick lead and has an unlimited range in air

The half-life of a radioactive substance is the time it takes:

• for the number and (therefore the mass) of parent atoms in a sample to halve
• for the count rate from the original substance to fall to half its initial level

The use we can make of a radioactive substance depends on: 

• its half-life and 
• the type of radiation it gives out

• Light from distant galaxy is red-shifted to longer wavelengths
• The further away the galaxy the bigger the red shift
• Red shift provides evidence that the univercse is expanding
• The universe started with the Big Bang, a massive explosion from a very small point

• Observations are made with telescopes that may detect visible or other electromagnetic radiations
• Observations of the Solar System and galaxies can be carried out from the Earth or from space