WAVES

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

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 

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 *****s changed when exposed to different colours of light
• Created a spectrum in a dark room using a light source and a prism - exposed *****s 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 ***** 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 

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 

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 

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 

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 

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

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

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 

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 

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 

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

• 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 

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 

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 

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  

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 

• 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