WAVES

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  • Created by: Sam
  • Created on: 30-05-13 13:58

WAVES - BASIC PRINCIPLES

WAVELENGTH, FREQUENCY, AMPLITUDE AND SPEED

  • WAVELENGTH: distance from one peak to the next
  • FREQUENCY: how many complete waves there are per second - measured in Hz, kHz (1 kHz = 1000 Hz) or MHz (1 MHz = 1 000 000 Hz)
  • AMPLITUDE: height of the wave (from mid-line to peak)
  • SPEED: how fast it travels
  • Waves transfer energy and information without transforming matter 

WAVE SPEED

  • Speed (m/s) = Frequency (Hz) X Wavelength (m) OR
  • v = f x λ
  • Wave speed (m/s) = distance (m)/ time (s)

TRANSVERSE/ LONGITUDINAL 

  • TRANSVERSE: vibrations at 90 degrees to DIRECTION OF TRAVEL - EM waves, S-waves
  • LONGITDUNIAL: vibrations along SAME DIRECTION as wave - sound, ultrasound, P-waves
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ELECTROMAGNETIC WAVES

SEVEN TYPES 

  • In order of increasing frequency and decreasing wavelength: RADIOWAVES, MICROWAVES, INFRARED, VISIBLE LIGHT, ULTRAVIOLET, X-RAYS, GAMMA RAYS 
  • All transverse waves 
  • All travel at the same speed in a vacuum 
  • EM waves with higher frequencies have shorter wavelength 

HERSCHEL'S DISCOVERY OF INFRARED RADIATION 

  • Discovered in 1800 with sunlight and a prism
  • When white light goes through a prism it creates a spectrum of colours - Herschel created it on a screen 
  • Measured temperature of each colour - increased from violet to red 
  • Measured temperature just past the red part of the spectrum with no visible light - had the highest temperature of all 
  • He had discovered infrared - an invisible type of radiation 
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ELECTROMAGNETIC WAVES (2)

RITTER'S DISCOVERY OF ULTRAVIOLET RADIATION 

  • Discovered in 1801
  • Ritter knew silver chloride turned from white to black when exposed to light - decided to measure how quickly silver chloride coated strips changed when exposed to different colours of light
  • Created a spectrum in a dark room using a light source and a prism - exposed strips of paper to each colour and timed how long they took to turn black 
  • Strips changed quickest when exposed to light nearer the blue end of the spectrum 
  • Placed a strip in the area just past the violet part - quickest change of all 
  • Ritter had discovered ultraviolet radiation - invisible form of light that exists on the other side of the visible spectrum 
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DANGERS OF ELECTROMAGNETIC RADIATION

PROPERTIES OF EM WAVES DEPEND ON THEIR FREQUENCY 

  • As frequency changes its interaction with matter changes - the way a wave's absorbed, reflected or transmitted by any given substance depends entirely on frequency 
  • EM waves at each end of the spectrum tend to be able to pass through material whilst those nearer the middle are absorbed 
  • Effects on humans depends on the frequency as this determines the energy of the waves
  • The higher the frequency the more energy and the more harmful 

MICROWAVES

  • Have a similar frequency to the vibrations of many molecules - increase these and heat - can HEAT HUMAN BODY CELLS
  • Mobile phones use microwaves - increasing use has caused concern as handset is held close to the brain - suggested with links with brain tumours 
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DANGERS OF ELECTROMAGNETIC RADIATION (2)

INFRARED

  • Makes surface molecules of any substance vibrate - has a heating effect
  • Has a higher frequency and more energy than microwaves
  • Too much exposure to infrared means SKIN BURNS

ULTRAVIOLET

  • More energy and higher frequency than infrared
  • 'Ionising' - carries enough energy to knowck electrons off atoms
  • SUNBURN happens when surface skin cells have been damaged by absorbing UV in sunlight
  • Can cause cell mutation or destruction, and SKIN CANCER
  • UV in sunlight can also cause EYE DAMAGE

GAMMA/ X-RAYS

  • Very high-frequency - also ionising with much more energy than UV rays 
  • Much more damaging and can penetrate further into the body - can cause CELL MUTATION or destruction, leading to tissue damage or CANCER 
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RADIO WAVES AND MICROWAVES

RADIOWAVES

  • Broadcast TV and radio signals and transmit satellite signals 
  • Long-wave radio (wavelengths of 1 - 10 km) transmitted around the world - long wavelengths bend around curved surface of Earth - around hills, into tunnels etc
  • TV and FM radio transmissions - very short wavelengths (10 cm - 10 m) - must be in direct sight of transmitter for reception - doesn't bend around hills or travel far through buildings
  • Short-wave radio signals and shorter medium-wave, depending on conditions and time of day, (10 m - 100 m) received at long distances - reflected from ionosphere - electrically charged layer in the Earth's upper atmosphere - some very short radio waves can pass through - satellite communications

MICROWAVES

  • Used for satellite communication - must use ones that can pass easily through the Earth's watery atmosphere
  • Satellite TV - signal from transmitter is transmitted into space and picked up by the satellite receiver dish orbiting 1000s km above Earth - then transmits signal back in a different direction - received by satellite dish on ground with slight time delay because of distance signals must travel 
  • Mobile phone signals also travel to the nearest transmitter as microwaves 
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RADIO WAVES AND MICROWAVES (2)

MICROWAVE OVENS

  • Microwaves need to be absorbed by water molecules in food to heat it up - different wavelength to those in satellite communication 
  • Penetrate up to a few centimetres into food before being absorbed by water molecules - energy from them causes food to heat up - heat energy then conducted/ convected to other parts of the food 
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INFRARED RADIATION

MONITORING TEMPERATURE

  • Also known as heat radiation, given out by all hot objects - the hotter, the more IR radiation given out
  • Can be used to monitor temperature - infrared sensors detect
  • Also detected by night-vision equipment - turns it into electrical signal, displayed on a screen as a picture - the hotter an object, the brighter it appears - used by police and military to spot people running away 

OPTICAL FIBRES

  • Can carry data over long distances as pulses of IR radiation 
  • Work by bounding waves off the sides of a thin inner core of glass or plastic - wave enters one end of fibre and is reflected repeatedly until it emerges at the other end 

AROUND THE HOME

  • Used in cooking - grills and toaster, using remote controls - transfer information to TVs and DVD players, used to transmit information between mobiles and computers - only over short distances, infrared sensors used in security systems - detect heat from intruder
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VISIBLE LIGHT, UV AND X-RAYS

VISIBLE LIGHT 

  • When light enters eye it gets refracted through lens and focused on retina at the back - retina then sends messages to the brain and it interprets them
  • Cameras use a lens to focus visible light onto a light-sensitive film or electronic sensor that records the image - aperture controls how much light enters, shutter speed determines how long it's exposed to light

ULTRAVIOLET

  • Fluorescence - UV radiation absorbed then visible light emitted - why fluorescent colours look so bright
  • Banks print special markings in fluorescent ink on bank notes to detect forgeries - under UV light, genuine notes display special markings but fake notes are often printed on cheaper paper that's slightly fluorescent so they glow all over with no markings - amount of radiation emitted being measured and detected
  • Fluorescent lamps use UV to emit visible light - safe to use as all UV's absorbed by phosphor coating on the inside of the glass
  • Security pens - mark property with name - ink only visible in UV light - helps police identify property if stolen
  • Can be used to disinfect water - kills off any viruses and bacteria in water, making it safer
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VISIBLE LIGHT, UV AND X-RAYS (2)

X-RAYS

  • Radiographers in hospitals take X-ray photographs to look for broken bones
  • Pass easily through flesh but not so easily through denser material like bones or metal - amount of radiation absorbed giving an image 
  • Can cause cancer so radiographers wear lead aprons and stand behind a lead screen or leave the room to keep their exposure to a minimum 
  • Used by airport security to scan luggage 
  • Some airports use X-ray scanners on passengers - low-level so less harmful than those in hospitals 
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GAMMA RAYS AND IONISING RADIATION

RADIOTHERAPY 

  • High doses of gamma rays will kill all living cells - used to treat cancers 
  • Rays must be directed carefully and at the right dosage to kill cancer cells without killing too many normal cells 
  • Fair bit of damage is done to normal cells - makes the patient feel very ill but if the cancer is successfully killed off it's worth it
  • Can also be used to diagnose cancer - radioactive isotope injected into patient - gamma camera then used to detect where it travels in the body, creating an image which can be used to detect where there might be cancer

STERILISATION

  • Food can be exposed to a high dose of gamma rays killing all microbes - keeps fresh for longer and perfectly safe to eat afterwards 
  • Medical instruments sterilised the same way 
  • Advantage of irradiation over boiling is it doesn't involve high temperatures - things can be totally sterilised without damaging them 
  • Radioactive isotope for this needs to be a very strong emitter of gamma rays 
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GAMMA RAYS AND IONISING RADIATION (2)

IONISING RADIATION 

  • The other two types are alpha and beta
  • Emitted all the time by radioactive sources when their nuclei decay 
  • Completely random and so can't be predicted for a given nucleus - when it does it'll spit out one or more of three types of ionising radiation 
  • All three types transfer energy - why they're ionising - so energetic that they bash into atoms and knock electrons off them 
  • All have their own uses but can also be very dangerous - if it enters your body it will collide with molecules in cells - causes ionisation, which damages or destroys the molecules - leads to cancer 
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LOOKING INTO SPACE

SPACE TELESCOPES 

  • Earth's atmosphere absorbs a lot of light from space before it can reach us - to observe the frequencies absorbed you have to go above the atmosphere 
  • Light pollution makes it hard to pick out dim objects, air pollution can reflect and absorb light from space - telescopes should be on top of mountains and far away from any cities for the best view 
  • To avoid atmosphere problems put the telescope in space - the Hubble space telescope was the first one launched by NASA in 1990 - can see objects that are a billion times fainter than unaided from Earth 

TELESCOPES AND EM WAVES

  • The earliest telescopes were optical telescopes - visible light - used to look at objects close by and in other galaxies but many objects aren't detectable using visible light - other types of EM telescopes needed to observe
  • Telescopes developed for all parts of EM spectrum from '40s onwards - see what we couldn't see before and learn more about the Universe
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LOOKING INTO SPACE (2)

  • X-ray telescopes - show violent, high-temperature events in space, like exploding stars
  • Radio telescopes - responsible for discovery of cosmic microwave background radiation 
  • Telescopes improving all the time - bigger ones give better resolution and can gather more light - we can see things that were too faint before, improved magnification means we can look further - more and more galaxies being discovered 
  • Important to help scientists learn about galaxies' life cycles - some pictures show them at different stages of life - used to help learn more about how they are formed and they evolve
  • Modern telescopes work alongside computers - help create clearer and sharper images and make it easy to capture these pictures so they can be analysed later - make it possible to collect and store huge amounts of data without relying on humans - make it easier and quicker to analyse all this data too 
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SPACE AND SPECTROMETRY

SPECTROMETERS

  • Tool used to analyse light given out by stars and galaxies 
  • Telescope directs beam of light into spectrometer and through a slit - diffracts the light and splits it up into a spectrum 
  • Light spectra from stars and galaxies contain dark lines - caused by light at those wavelengths being absorbed - called absorption spectra 
  • Can be used to work out what the stars and galaxies are made of - each element has own absorption spectrum 
  • Some spectra have bright lines - emission spectra - lines caused by extra light being emitted at those wavelengths - can also be used to work out what something is made of
  • The spectra for galaxies further away appear more red than they should 
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ULTRASOUND AND INFRASOUND

ULTRASOUND

  • Sound waves beyond the range of human hearing - above 20 000 Hz
  • Partial reflection - wave passes from one medium to anoher, some is reflected off the boundary, and some is transmitted (and refracted) 
  • So, whenever ultrasound is pointed at an object with boundaries some ultrasound will get reflected back
  • The time it takes can be used to measure how far away the boundary is 

USES OF ULTRASOUND

  • Pre-natal scanning of a foetus: 
    • Waves can pass through the body but some is reflected back and detected whenever they meet a boundary 
    • The exact timing and distribution of these echoes are processed by a computer, producing a video image 
  • Nobody knows for sure whether it's safe in all cases but X-rays would definitely be dangerous 
  • Sonar:
    • Used by boats and submarines to detect things in the water around them 
    • Emit waves of ultrasound which reflect off things and detect these as they arrive back at he boat
    • Computers on-board time the delay between emitting and detecting and then calculate how far away
    • Animals like bats and dolphins use ultrasound to sense their way around - many also use it to communicate with one another 


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ULTRASOUND AND INFRASOUND (2)

INFRASOUND

  • Sound with frequency below range of human hearing - below 20 Hz
  • Waves have long wavelengths - can travel long distances and diffract around objects easily
  • Elephants use to communicate with other members of herd over long distances
  • Tigers use in growls and roars so they can be heard by rivals/ mates from far away
  • Some microphones sensitive enough to detect and can be used to monitor animal movements
  • Meteor strikes and volcanic eruptions produce it - detecting  
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THE EARTH'S STRUCTURE

BASIC STRUCTRE

  • Crust - bit we live on, very thin
  • Mantle - has all properties of a solid but can flow very slowly, radioactive decay happens withing - produced heat making it flow in convection currents
  • Core - believe made of iron and nickel, inner core solid but outer core liquid

TECTONIC PLATES

  • Large pieces of crust and upper part of mantle
  • Drift because of convection currents - mostly by a few cm per year 
  • May slide past each other at boundaries - sometimes causes earthquakes 

EARTHQUAKES AND TSUNAMI WAVES 

  • Frequent in some countries - can be extremely destructive, especially where no suitable housing - would be useful to predict but very difficult 
  • Probabilities based on previous occurrences used to predict - gives time to prepare 
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SEISMIC WAVES

  • Caused by earthquakes - detected all over planet's surface using seismometers
  • Seismologists work time it takes to reach each seismometer
  • Also note which parts of Earth don't receive shock waves 
  • P-waves: longitudinal, travel through solids and liquids, faster than S-waves 
  • S-waves: transverse, only travel through solids, slower 
  • When reaching a boundary some waves reflected 
  • Also change speed as properties of mantle and core change - causes change in direction (refraction)
  • Change speed gradually - curved path, but path has a kink with sudden changes 
  • By observing this scientists worked out where properties of Earth change dramatically - current understanding of internal structure basis 

SEISMOMETERS 

  • Readings used to work out epicentre 
  • Waves travel at different speeds so see two tremors seen on seismogram 
  • Using time difference between waves - distance 
  • Circle can be drawn and centred on location of seismometer - distance radius - distance arc
  • Three or more arcs cross at one place - epicentre - method = triangulation 
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