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OCR 21st Century Science P2

In a general model of radiation: one object (a []) emits [] source--radiation--; the radiation travels [] from the source and can be []ed, []ed, []ed (or a [] of these) by [] it encounters outwards--reflected, transmitted or absorbed--combination--materials--; radiation may affect an object (a []) some distance away, when it is [] detector--absorbed.
Light is one of a family of [] called the [] [] radiations--electromagnetic spectrum--.
A beam of electromagnetic radiation transfers energy in 'packets' called photons.
The higher the frequency of an electromagnetic radiation, the more [] is transferred in each [] energy--photon--.
The electromagnetic radiation (in order of frequency): radio waves, microwaves, infrared, (^red)visible light(^violet), ultraviolet, X-rays, gamma rays. The bracketted items denote that the visible light spectrum goes from [] to [] red to violet.
All types of electromagnetic radiation travel at exactly the same [] through a vacuum in space speed - [] km/s 300,000,
Intensity of electromagnetic radiation |on a unit area| = number of photons per second x energy of each photon, hence the e[] arriving at a square metre of surface each second is a useful measure of intensity energy--.
The energy transferred to an absorber by a beam of electromagnetic radiation depends on both the [] of photons and each photon's [] number--energy--.
intensity ∝ 1/distance², the distance being between the [] and the [] source--detector--, this is as the area that the energy is spread over is proportional to the distance; the larger the area, the less the [] per unit area, hence the less the [] energy--intesity--.
As intensity ∝ 1/distance², as distance (from the source to the detector) increases, the intensity decreases. The reason is fractionally due to the fact that there is partial [] of radiation by the [] it travels through absorption--medium--, and the absorbed amount increases with the [] that the radiation travels distance--.
Some electromagnetic radiations (ultraviolet radiation, X-rays, gamma rays) have enough [] to change [] or [] energy--atoms or molecules--, which can intiate chemical reactions.
High energy ultraviolet raidation, X-rays and gamma rays can cause ionisation.
Which electromagnetic radiations have enough energy to change atoms or molecules? ultraviolet, X-rays, gamma rays.
The electromagnetic radiations which are ionising are those with high enough [] energy to [] an [] from an atom or molecule photon--remove--electron--: this is known as ionisation.
The h[] effect of [] radiation can damage [] cells heating--absorbed--living--.
All forms of electromagnetic radiation deliver e[] energy; this will [] the material that absorbs the radiation heat--. The amount of heating is proportional to the [] of the radiation and the [] of [] the radiation is absorbed for intensity--length--time--.
Some poeple have concerns about health risk from low [] [] radiation intensity microwave--, for example from m[] [] []sets and m[], mobile phone handsets and masts, but the evidence for this is disputed.
Some m[] are strongly absorbed by water molecules and so can be used to [] objects containing water microwaves--heat--.
The metal cases and door screens of microwave ovens [] or [] microwaves radition and so protects users from the radiation reflect--absorb--.
Some materials (i.e. radioactive materials) emit [] [] radiation [] of the time ionising gamma--all--.
With increased exposure to ionising radiation, [] to [] cells increases, damage--living--, enventually leading to [] or [] death cancer--cell--.
The [] layer absorbs ultraviolet radiation emitted by the [] ozone--Sun--, producing [] changes in that part of the [] chemical--atmosphere--.
As the ozone layer absorbs some ultraviolet radiation emitted by the Sun, it protects living organisms from some of the radiation's [] effects harmful--.
Sun-screens and clothing can be used to [] some of the ultraviolet radiation from the Sun absorb.
Physical barriers [] some ionising radiation absorb--, for example: X-rays are absorbed by [] materials dense--so can be used to show shadow pictures of []s in our body or of objects in aircraft passengers' [] bones--luggage--, and radiographers are protected from radiation by [] materials such as [] and [] dense--lead--concrete--.
All object emit [] radiation with a [] frequency that increases with [] electromagnetic--principal--temperature--.
The Earth is surrounded by an [] which allows some of the [] radiation emitted by the [] to pass through atmosphere--electromagnetic--Sun--.
The radiation emitted by the Earth, which has a []er p[] f[] than that emitted by the Sun lower principal frequency--, is [] or [] back by some [] in the atmosphere absorbed--reflected--gases--; this keeps the Earth []er than it would otherwise be and this is called the [] [] warmer--greenhouse effect--.
One of the main g[] gases in the Earth's [] is carbon dioxide gases--atmosphere--, which is present in very [] amounts small--.
Apart from carbon dioxide, other greenhouse gases include m[] which is present in very [] amounts, and [] v[] methane--small--water vapour--.
The amount of carbon dioxide in the atmosphere is controlled by the carbon cycle. Processes that remove carbon dioxide from the air: [] by plants, [] in the oceans photosynthesis--dissolving--. Processes that return carbon dioxide to the air: [] by plants, animals and microbes; c[] (i.e. [] wood and fossil fuels such as coal, oil and gas) respiration--combustion--burning--; carbon dioxide is also given out by t[] d[] of limestone, like in the manufacture of iron, steel and cement thermal decomposition--.
For thousands of years, the processes in the carbon cycle were c[] constant, so the percentage of carbon dioxide in the atmosphere did not change.
Over the past 200 years, the percentage of carbon dioxide in the atmosphere has [] steadily because humans are: [] more and more fossil fuels as [] sources increased--burning--energy--; and [] large areas of forest to [] land, meaning that there is less [] removing carbon dioxide from teh air burning--clear--photosynthesis--.
Computer generated [] models, based on diffent amounts of [] [] in the atmosphere, have produced the same changes as have been [] in the real world climate--carbon dioxide--observed--. The computer climate models provide [] that [] activities are causing global warming evidence--human--.
Due to global warming, it may become impossible to continue growing some food crops in particular regions because of climate change.
Global warming may result in more e[] weather events extreme--, due to increased c[] and larger amounts of [] [] in the []er atmosphere convection--water vapour--hotter--.
One consequence of global warming could be the [] of low lying lands due to rising [] levels flooding--sea--, caused by []ing c[] [] and expansion of [] in the oceans melting continental ice--water--.
The electromagnetic radiation of some frequencies can be used for transmitting i[] information, since: (a)some [] waves and []s are not strongly [] by the atmosphere radio--microwaves--absorbed--, so can be used to carry [] for radio and TV programmes information-- (b)l[] and [] radiation can be used to carry information along [] fibres because the radiation travels large distances through [] without being significantly [] light--infrared--optical--glass--absorbed--.
Information can be superimposed onto an [] c[] wave, to create a [] electromagnetic carrier wave--signal--.
A signal that can vary continuosly is called an analogue signal.
A signal that can only take a small number of discrete values (usually two) is called a digital signal.
Sound and images can be transmitted d[] (as a [] signal) digitally--digital--.
In digital transmission, the digital code is made up from just two symbols - '[]' and '[]' 0--1--. The coded information can be carried by switching the [] [] wave [] and [] to produce short burst of [] (p[]) where '0' = no [] and '1' = [] electromagnetic carrier--on--off--waves--pulses--pulse--pulse--.
When the aforementioned waves are received, the p[] are [] to produce a [] of the original sound wave or image pulses--decoded--copy--.
An important advantage of [] signals over [] signals is that if the original signal has been affected by noise it can be recovered more easily digital--analogue--. This is because the noise is usually lower in [] than the [] states of the digital signal amplitude--'on'--, hence the electronics in the amplifiers can [] the noise and it doesn't get [] along ignore--passed--. As a result digital signals maintain their q[] better than analogue signals quality--.
The amount of information needed to store an image or sound is measured in [] ([]) bytes (B).
Generally, the more information stored, the higher the [] of the sound or image quality--.
An advantage of using digital signals is that it can s[] and p[] by computers stored--processed--.
Created by: Toluo
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