28/05/13Some definitions…Some definitions…1) Amplitude – this is“how high” the wave is:2) Wavelength (λ) – this is thedistance between twocorresponding points on thewave and is measured in metres:3) Frequency – this is how many waves pass byevery second and is measured in Hertz (Hz)
28/05/13Some definitions…Some definitions…Transverse waves arewhen the displacementis at right angles to thedirection of the wave…Longitudinal wavesare when thedisplacement isparallel to thedirection of the wave…
28/05/13The Wave EquationThe Wave EquationThe wave equation relates the speed of the wave to itsfrequency and wavelength:Wave speed (v) = frequency (f) x wavelength (λ)in m/s in Hz in mVλf
28/05/13Some example wave equation questionsSome example wave equation questions1) A water wave travels through a pond with a speed of1m/s and a frequency of 5Hz. How far apart are thewaves?2) The speed of sound is 330m/s (in air). When Rickyhears this sound his ear vibrates 660 times a second.What was the wavelength of the sound?3) Purple light has a wavelength of around 6x10-7m. If itsfrequency is 5x1014Hz what is the speed of light?4) Red light travels at the same speed. Work out its
28/05/13Properties of LightProperties of LightLight travels in straight lines:Laser
28/05/13Light travels VERY FAST – around300,000 kilometres per second.At this speed it cango around the world 8times in one second.
28/05/13Light travels much faster than sound. Forexample:1) Thunder and lightningstart at the same time,but we will see thelightning first.2) When a starting pistolis fired we see thesmoke first and thenhear the bang.
28/05/13We see things because theyreflect light into our eyes:Homework
28/05/13Luminous and non-luminous objectsA luminous object is one that produces light.A non-luminous object is one that reflects light.Luminous objects Reflectors
28/05/13ShadowsShadowsShadows are places where light is “blocked”:Rays of light
28/05/13ReflectionReflectionIncident rayNormalReflected rayAngle ofincidenceAngle ofreflectionMirrorAngle of incidence = Angle of reflectionAngle of incidence = Angle of reflection
28/05/13Using mirrorsUsing mirrorsTwo examples:1) A periscope2) A car headlight
28/05/13Refraction through a glass block:Refraction through a glass block:Wave slows down and bendstowards the normal due toentering a more dense mediumWave speeds up and bendsaway from the normal due toentering a less dense mediumWave slows down but isnot bent, due to enteringalong the normal
28/05/13RefractionRefractionRefraction is when waves ____ __ or slow down due totravelling in a different _________. A medium issomething that waves will travel through. When a pen isplaced in water it looks like this:In this case the light rays are slowed down by the waterand are _____, causing the pen to look odd. The twomediums in this example are ______ and _______.Words – speed up, water, air, bent, medium
28/05/13Finding the Critical Angle…Finding the Critical Angle…1) Ray gets refracted4) Ray getsinternally reflected3) Ray still gets refracted (just!)2) Ray still gets refractedTHE CRITICALANGLE
28/05/13Uses of Total Internal ReflectionUses of Total Internal ReflectionOptical fibres:An optical fibre is a long, thin, _______ rod made ofglass or plastic. Light is _______ reflected from oneend to the other, making it possible to send ____chunks of informationOptical fibres can be used for _________ by sendingelectrical signals through the cable. The main advantageof this is a reduced ______ loss.Words – communications, internally, large, transparent, signal
28/05/13Other uses of total internal reflectionOther uses of total internal reflection1) Endoscopes (a medical device used to see inside the body):2) Binoculars and periscopes (using “reflecting prisms”)
28/05/13DiffractionDiffractionMore diffraction if the size of the gap is similar to the wavelengthMore diffraction if wavelength is increased (or frequency decreased)
28/05/13Sound can also be diffracted…Sound can also be diffracted…The explosion can’t be seen over the hill, but it can beheard. We know sound travels as waves because soundcan be refracted, reflected (echo) and diffracted.
28/05/13Diffraction depends on frequency…Diffraction depends on frequency…A high frequency (short wavelength)wave doesn’t get diffracted much – thehouse won’t be able to receive it…
28/05/13Diffraction depends on frequency…Diffraction depends on frequency…A low frequency (long wavelength) wavewill get diffracted more, so the housecan receive it…
28/05/13Deviation of LightDeviation of LightThis happens because the light is refracted:RED LIGHT isrefracted THELEASTPURPLE LIGHT isrefracted THE MOST
28/05/13The electromagnetic spectrumThe electromagnetic spectrumGamma rays X-rays Ultra violet Visible light Infra red Microwaves Radio/TVEach type of radiation shown in the electromagnetic spectrum has adifferent wavelength and a different frequency:Each of these types travels at the same speed through a _______(300,000,000m/s), and different wavelengths are absorbed by differentsurfaces (e.g. infra red is absorbed very well by ___________ surfaces).This absorption may heat the material up (like infra red and _______) orcause an alternating current (like in a __ _______).Words – black, microwaves, long, short, TV aerial, vacuumHigh frequency,_____ wavelengthLow frequency, _____(high) wavelengthγ
28/05/13Using an oscilloscopeUsing an oscilloscope1) Quiet sound, low frequency:2) Quiet sound, high frequency:3) Loud sound, low frequency:4) Loud sound, high frequency:
28/05/131) Pre-natal scanning2) Cleaning – it can be used todislodge dirt3) Detecting flaws or cracks4) Medical treatmentUltrasoundUltrasoundUltrasound is the region of sound above 20,000Hz – itcan’t be heard by humans. There are a number ofuses for ultrasound:
28/05/13How does ultrasound work?How does ultrasound work?Ultrasonic waves are partly _________ at the boundaryas they pass from one _______ to another. The timetaken for these reflections can be used to measure the_______ of the reflecting surface and this information isused to build up a __________ of the object.Words – depth, reflected, picture, medium
28/05/13IonosphereTransmitting informationTransmitting informationAs we said before, different types of electromagnetic radiation can beused to send different types of information, e.g. an optical fibre:Optical fibres have two main advantages: they can send more informationcompared to electrical cables of the same diameter and with less signalweakening.Another example is radio waves:The longer wavelength waves are transmitted by being reflected off theionosphere (an electrically charged layer in the Earth’s atmosphere).
28/05/13Transmitting informationTransmitting informationSounds can be transmitted long distances by converting them intoelectrical signals first:Radio waves can also be used as “carrier waves”:Signal ____ wave ______ wave TransmitterWave isdemodulatedback into a ____Words – signal, frequency, modulated, carrier, sound, amplifiedA microphone converts ______ waves into electricalsignals with the same amplitude and ________. Thesewaves are then transmitted through cables where theyare regularly _________ to reduce signal loss.
28/05/13Analogue vs. DigitalAnalogue vs. DigitalAnalogue signals (like talking ormusic) continually vary inamplitude and/or frequency+-10Digital signals, however, are eitheroff or on, and the information is sentin a series of pulsesThere are two main advantages of digital:1) More information can be sent down the same cable2) Better quality, because a digital signal can be amplified withoutamplifying the extra noise:
28/05/13The Structure of the EarthThe Structure of the EarthA thin crust -10-100km thickA mantle – has theproperties of a solidbut it can also flowA core – made ofmolten nickel and iron.Outer part is liquidand inner part is solidHow do we know this? These facts have all beendiscovered by examining seismic waves (earthquakes)
28/05/13Seismic wavesSeismic wavesEarthquakes travel as waves through the Earth – we call themSEISMIC WAVES. There are two types:P waves:1) They are longitudinal so they cause the groundto move up and down2) They can pass through solids and liquids3) They go faster through more dense materialS waves:1) They are transverse so they cause the groundto move from right to left2) They ONLY pass through solids3) They are slower than P waves4) They go faster through more dense material
28/05/13Seismic wavesSeismic wavesP waves travel throughthe Earth and arerefracted when theypass through a mediumS waves will only travelthrough a solidThe paths of thesewaves are all curvedbecause density isgradually changingThese observations tell us 3 things about the Earth: 1) It has a thincrust, 2) it has a semi-fluid mantle where density increases with depth,3) a core with a liquid outer part and a solid inner part.