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Modute 12

WAVES AND RADIATION

Summary for Pupils

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Module 12

Waves and Radiation

12.1 Light and Sound Waves12.2 Electromagnetic Spectrum12.3 Radioactivity12.4 Radioactive decay12.5 Sound and Ultrasound12.6 Seismic Waves

Using this booklet

This booklet provides you with a set of learning objectives for theModule 12 of your Science course. There are six sections, as listed above,and each has a set of numbered objectives. The booklet is not areplacement for your text books and notes. Use this booklet as you are

learning the topics and for revision to check if you understand the essentialpoints in the module. Take action if you are not sure about any of them byreading your notes, finding out more from text books or information oncomputer and by consulting your teacher. There is a table at the back ofthe booklet for you keep a record of the objectives you have learned. If youuse it you will know which ones need further study.

Foundation and Higher Papers

There are two tiers for the papers which the examination board provide fortesting candidates. They are called Foundation and Higher. When thetime comes, your teacher will select the paper which suits you best. Try tolearn as many of the objectives given in this booklet as you are able. Yourend of course exam will include this particular Module (Waves andRadiation). All candidates study the general objectives. The more difficultobjectives which will only be tested on the higher paper are labelled with aH on the objective number, in bold and underlined (e.g. 14H.). Try tomaster these as well.

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Unit 12.1 Licrht and Sound Waves

Water waves, and other waves that we can see, behave in the same way

as light and sound. This suggests that light and sound might also travelas waves.

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Echoes. Sounds bounce back (reflect) fromEchoes are sound reflections.

hard surfaces.

Light reflection. When a ray of light is reflected from a flat, shinysurface (plane mirror) the angle at which it leaves the surface is

the same as the angle at which it meets the surface.

Reflection diagrams. Candidates should be able to show on adiagram how rays of light are reflected by a plane mirror.

Light refraction. Rays of light change direction (are refracted)when they cross the boundary between one transparent substanceand another, unless they meet the boundary at right angles (alongthe normal).

Refraction diagrams. Candidates should be able to show on adiagram what happens to rays of light when they cross theboundary between air and glass (or Perspex or water) in eitherdirection.

Refraction of sound. Like light, sounds are refracted (i.e. theirdirection is changed when they cross the boundary between twodifferent substances at an angle other than a right angle).

Waves. Waves can be produced on ropes and springs and onthe surface of water. They set up a regular pattern of disturbances.

Wave words.Amplitude - the maximum disturbance caused by a waveWavelength - the distance between a particular point on onedisturbance and the same point on the nextFrequency - the number of waves each second produced by asource (or passing a particular point); it is measured in hertz (Hz).

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9 .

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I I .

12.

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Energy. Waves transfer energy from a source to other placeswithout any matter being transferred.

Wave reflection. Waves travelling along a rope or spring, or

across the surface of water, can be reflected.

Wave refraction. Waves travelling across the surface ofwater can be refracted.

Water wave refraction. If the speed of a water wave changeswhen it crosses the boundary between two regions then there willalso be a change in the direction of the wave (refraction) unlessthe direction of the wave is along the normal.

Light and sound. The behaviour of waves suggests that light andsound:- travel as waves- are refracted because they travel at different speeds in

different substances (mediums).

Internal reflection. When a ray of light travels from glass,Perspex, or water into air, some of the light is also reflected from

the boundary.

normal ! air

angles: r = i (reflection)R > i (refraction, glass to air)

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Unit 12.2 Electromaanetic spectrum

White light can be split up into a spectrum of different colours. There are

many other types of radiation beyond the ends of the visible spectrum.Each type of radiation in this extended spectrum, known as theelectromagnetic spectrum, has its own special properties and uses.

1 . Prisms. When rays of light pass through prisms their directionmay be changed. Candidates should be able to show on adiagram how a ray of light can be deviated by a triangular prism.

2. White light. When white light is passed through a prism a

spectrum of colours is produced. This is because white light ismade up of many different colours and different corours of light arerefracted by different amounts; red light is refracted the least andviolet the most. Candidates should be able to draw a diagram toshow how a prism disperses white light into a spectrum.

3 . Electromagnetic radiation. Visible light is one type ofelectromagnetic radiation, The various types of electromagneticradiation form a continuous spectrum extending far beyond each

end of the visible spectrum:

highestfrequency

shortestwavelength

lowestfrequency

gamma raysX raysultraviolet raysvisible light

infra red raysmicrowavesradio waves

longestwavelength

4. Speed. All types of electromagnetic radiation travel at thesame speed in space.

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5. Substances. Different wavelengths of electromagneticradiation are reflected, absorbed or transmitted differently bydifferent substances and types of surface.

6.

Absorption. When electromagnetic radiation is absorbed, theenergy it carries: - makes the substance which absorbs it hotter- may create an alternating current with the samefrequency as the radiation itself.

The uses and effects of different types of radiation depend onthese and other properties,

7. Radio waves. Radio waves are used to transmit radio and TVprograms between different points on the Earths surface. Longerwavelength radio waves are reflected from an electrically chargedlayer in the Earths upper atmosphere. This enables them to besent between distant points despite the curvature of the Earthssurface.

8. Microwaves. Microwave radiation of wavelength which canpass easily through the Earths atmosphere is used to sendinformation to and from satellites. Microwave radiation, with

wavelengths strongly absorbed by water molecules, is used forcooking.

9. lnfra red. lnfra red radiation is used in grills, toasters andradiant heaters, in optical fibre communication and for the remotecontrol of TV sets and VCRs.

IO. Visible light. Visible light is not only used for normal seeing butcan also be sent along optical fibres, for example, in endoscopes

used by doctors to see inside patients bodies.

11. Optical fibres. When light travels down an optical fibre, allthe light may stay inside the fibre until it emerges from the otherend. This is because light travels down the fibre by repeated totalinternal reflection.

12. Total internal reflection. When a ray of light hits theinside surface of glass, Perspex or water it will all be reflected back

inside the material if the angle between the incident ray and thenormal is greater than a certain angle, called the critical angle.

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13. Information transmission. More information can be carriedby an optical fibre than by electrical signals through a cable of thesame thickness. There is also less weakening of the signal inoptical fibres.

14. Other uses. Candidates should be able to describe, using asuitable diagram, one other use of total internal reflection (such asin bicycle reflectors or in glass prisms in binoculars andperiscopes).

15. Ultraviolet radiation. Ultraviolet radiation is used toproduce a suntan in sunbeds. Special coatings which absorbultraviolet radiation and emit the energy as light are used in

fluorescent lamps and for security coding.

16. X rays. X rays are used to produce shadow pictures ofmaterials which X rays do not easily pass through, including bonesand metals.

17. Gamma rays. Gamma radiation is used to:

- kill harmful bacteria in food-sterilise surgical instruments- kill cancer cells.

18. Living cells. Different types of electromagnetic radiationhave different effects on living cells:

- microwaves are absorbed by the water in cells, which may bedamaged or killed by the heat released

- infra red radiation is absorbed by the skin and felt as heat- ultraviolet radiation can pass through the skin to deeper tissue.The darker the skin, the more ultraviolet it absorbs and the lessreaches deeper tissues

- X rays and gamma rays mostly pass through soft tissues, butsome is absorbed by the cells.

19. Danger. High doses of ultraviolet radiation, X radiationand gamma radiation can kill normal cells. Lower doses of thesetypes of radiation can cause normal cells to become cancerous.

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Unit 12.3 Radioactivity

Radioactive substances, which emit radiation all the time, are very usefulbut also very dangerous. It is important to understand the properties ofthe different types of radiation they emit.

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Radioactive substances. Some substances give outradiation all the time, whatever is done to them. These substancesare said to be radioactive.

Types of radiation. There are three types of radiationemitted by radioactive sources. They are called alpha (a), beta

and gamma (y) radiation.a9

Absorption. As radiation passes through a material it can beabsorbed. The greater the thickness of material the greater theabsorption. The absorption of radiation can be used to monitorand control the thickness of materials.

Ranges. - Alpha (a) radiation is easily absorbed by a fewcentimetres of air or a thin sheet of paper.

- Beta fp) radiation easily passes through air or a thinsheet of paper but is mostly absorbed by a fewmillimetres of metal.

- Gamma (r> radiation is very penetrating and requiresmany centimetres of lead or metres of concrete toabsorb most of it.

Background radiation. There are radioactive substances allaround us, including in the ground, in the air, in building materials

and in food. Radiation also reaches us from space. The radiationfrom all these sources is called background radiation.

lonisation. When radiation from radioactive materialscollides with neutral atoms or molecules these may becomecharged (ionised).

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Unit 12.3 Radioactivity

Radioactive substances, which emit radiation all the time, are very usefulbut also very dangerous. It is important to understand the properties ofthe different types of radiation they emit.

1.

2.

3.

4.

5.

6.

Radioactive substances. Some substances give outradiation all the time, whatever is done to them. These substancesare said to be radioactive.

Types of radiation. There are three types of radiationemitted by radioactive sources. They are called alpha (a), beta

and gamma (y) radiation.a9

Absorption. As radiation passes through a material it can beabsorbed. The greater the thickness of material the greater theabsorption. The absorption of radiation can be used to monitorand control the thickness of materials.

Ranges. - Alpha (a) radiation is easily absorbed by a fewcentimetres of air or a thin sheet of paper.

- Beta fp) radiation easily passes through air or a thinsheet of paper but is mostly absorbed by a fewmillimetres of metal.

- Gamma (r> radiation is very penetrating and requiresmany centimetres of lead or metres of concrete toabsorb most of it.

Background radiation. There are radioactive substances allaround us, including in the ground, in the air, in building materials

and in food. Radiation also reaches us from space. The radiationfrom all these sources is called background radiation.

lonisation. When radiation from radioactive materialscollides with neutral atoms or molecules these may becomecharged (ionised).

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2. Rutherfords experiments. Candidates should be able toexplain how the scattering experiments by Rutherfords teamprovided evidence for the nuclear model of the atom.

3.

Plum pudding model. Candidates should be able todescribe the plum pudding model of the atom which wasreplaced by Rutherfords nuclear model.

4. Mass and charge. The relative masses of protons, neutronsand electrons and their relative electric charges are as shown:

Mass ChargeProton I +INeutron I 0Electron negligible -1

5. Neutral atom. In an atom, the number of electrons is equal tothe number of protons in the nucleus. The atom as a whole has noelectrical charge.

6 . Proton number. All atoms of a particular element have thesame number of protons. Atoms of different elements have

different numbers of protons.

7. Nucleon number. The total number of protons and neutrons(nucleons) in an atom is called its mass (nucleon) number.8. Isotopes. Atoms of the same element which have different

numbers of neutrons are called different isotopes of the element.

9 . Nuclear changes. Radioactivity occurs as a result of changesin the nuclei of atoms. Radioactive isotopes are atoms withunstable nuclei.

IO. Nuclear disintegration. When an unstable nucleus splits up(disintegrates) it emits radiation and a different atom, with adifferent number of protons, is formed.

1 I. Dating. The older a radioactive material, the lessradiation it emits. *This idea can be used to date materials.

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12H. Radioisotopes. Any isotope of an element which isradioactive is called a radioisotope (radionucide).

13H. Alpha radiation. Alpha radiation consists of helium nuclei,particles made up of two protons and two neutrons.

14H. Beta radiation. Beta radiation consists of electrons emittedfrom the nuclei of atoms. For each electron emitted, a neutron inthe nucleus changes into a proton.

15H. Gamma radiation. Gamma radiation is very short wavelengthelectromagnetic radiation.

Nuclear fission.16H. Nuclear reactors use a process callednuclear fission. When an atom with a very large nucleus isbombarded with neutrons:

- the nucleus splits into two smaller nuclei- further neutrons are released which may cause further

nuclear fission resulting in a chain reaction- the new atoms which are formed are themselvesradioactive.

[Details of nuclear reactors are not required.]

Nuclear energy.17H. The energy released by an atom duringradioactive disintegration or nuclear fission is very large comparedto the energy released when a chemical bond is made betweentwo atoms.

18H. Uranium decay. Uranium isotopes, which have a very longhalf-life, decay via a series of relatively short-lived radioisotopes to

produce stable isotopes of lead. The relative proportions ofuranium and lead isotopes in a sample of igneous rocks can,therefore, be used to date the rock.

19H. Other dating. The proportions of the radioisotopepotassium-40 and its stable decay product argon can also be usedto date igneous rocks from which the gaseous argon has beenunable to escape.

20H. Calculations.Candidates should be able to make such dating

calculations when provided with appropriate data.

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Unit 12.5 Sound and Ultrasound

We use our ears to hear sounds, but just as there is electromagneticradiation with frequencies we cannot see, there are sound waves with

frequencies we cannot hear. These ultrasounds have several importantuses.

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6H.

7H.

8H4

Vibrations. Sounds are produced when objects vibrate. Thegreater the size (amplitude) of vibrations the louder the sound.

Frequency. The number of complete vibrations each secondis called the frequency (hertz, Hz). The higher the frequency of asound the higher its pitch.

Oscilloscope. Candidates should be able to compare theamplitudes and frequencies of sounds as they would appear on anoscilloscope trace.

Ultrasound. Electronic systems can be used to produceelectrical oscillations with any frequency. These electricaloscillations can be used to produce ultrasonic waves which have afrequency higher than the upper limit of the hearing range for

humans.

Uses. Ultrasonic waves can be used:- in industry for cleaning and for quality control- in medicine for pre-natal scanning.

Cleaning.cleaning delicatethem.

Ultrasonic waves in liquids can also be used formechanisms without having to disassemble

Reflections. Ultrasonic waves are partly reflected when theymeet a boundary between two different mediums, The time takenfor the reflections of ultrasonic pulses to reach a detector (usuallyplaced near to the source) is a measure of how far away such aboundary is.

Display. Reflection of ultrasound is used in industry todetect flaws in metal castings and in medicine for pre-natal scans.

Information about the time taken for reflections to travel is usuallyprocessed to produce a visual display.

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