so let's get started with unit 3 now I've split unit three into two halves or two parts essentially the first part of unit 3 will cover uh the general properties of a wave so what's a wave frequency wavelength uh reflection refraction defraction etc etc and we'll talk about sound waves and the electromagnetic spectrum so anything that doesn't have to do with light specifically part two will'll deal with light okay now before we continue just this is a friendly reminder every beginning of a chapter like when we start the new chapter you will find this big checklist this checklist is what the curriculum wants you to know like every single point is something you need to know so use that to check if you understood what you've revised or not so for example if you read recall and use the equation V equal F Lambda you know how to use that formula so that's fine you give it a good check but if you read this know that for a longitudinal if the direction of vibration is parallel to the direction of propagation and you realize you don't understand what's going on go back and revise longitudinal waves for example okay that's how to use this bit so let's get started with the very first thing General properties of a wave as in everything that's General about waves so whether it's the types of waves the properties that we measure in a wave the behavior of waves so reflection reflection defraction they apply to all waves so if it's slight sound other electromagnetic waves you waves in a rope water waves it applies to everything it's the most important bit of this chapter so let's first start by defining a wave what is a wave a wave is defined as the transfer of energy energy is being transferred now without transferring matter that's because molecules vibrate in a wave they don't move along with a wave when it comes to waves we have four types the first two types are opposites the next two types are also opposites the first two types are mechanical and electromagnetic as in does a wave need a medium to go through or does it not need a medium to go through so things like sound waves are mechanical things like light waves are electromagnetic because they do not need a medium to travel through then the other two opposites have to do with the direction of vibration of the molecules in a transverse wave the particles of that medium vibrate perpendicular to the direction of wave motion so if you have a wave moving to the right like in this transverse wave each molecule either moves up or down in a specific order so the particles vibrate up and down and that creates your transverse wave the peak is called a crust and the bottom Peak is called the truff the center line is often called the mean position the mean or average or original position then what's a longitudinal wave again they're opposite so if a transverse wave was where the molecues vibrated perpendicular a longitudinal is where they vibrate parallel to wave motion left and right forward and backwards so they vibrate parallel to wave motion propagation means motion instead of creating crests and troughs you have compressions and you have rare fractions each line here represents a line of atoms or molecules so when we draw the lines close to each other that's a compression this means the lines means molecules are super close to each other when we draw these lines far apart the molecules are far away these are not wave fronts we'll discuss wave fronts in a bit now transverse waves are all electromagnetic waves water waves and so on longitudinal waves are just sound but there's another type of wave called a seismic wave earthquakes and during earthquakes you actually have two types of waves that occur so zoom in here with me for a second a seismic wave again is basically an earthquake when an earthquake occurs like let's say here it actually emits two types of waves a wave that you know gets transmitted on the surface of the ground and a wave that travels underground now the underground wave is called a p-wave or primary wave sometimes known as a pressure wave this is longitudinal and it shovels very quickly waves on the surface however the surface vibrates up and down perpendicular to wave motion so it's a transverse wave so it's called an swaave surface wave right or secondary wave so keep in mind during an earthquake both longitudinal and transverse waves are produced we just need to remember which is which so p is primary s is secondary s is on the surface p is underground okay next what about the features of O like the properties that we measure in a wave you have first what we call a wave front when we start drawing waves to simplify our diagram I forgot how to word there to simplify our diagrams diagrams a wave is drawn using a ray which shows you the direction of wave motion and we draw wave fronts which are lines that are perpendicular to the ray perpendicular to the ray these essentially show you where a Crest is if it's a transverse wave or a compression is so if this is technically a Crest and this is also technically a Crest the distance between them will be the wavelength of the wave so keep that in mind now sometimes these wave fronts will be drawn circular so if it's a circular wave we draw just circles and the ray in this particular scenario will be drawn like so okay next oh let's go back to Red for a second let's continue so what is a wavefront it's just a line that represents the crest of a wave the distance between two wave fronts is the wavelength of the wave what is amplitude it's defined as the maximum displacement moved by the particles from their mean position or moved away from the mean position so when the molecules move up or down that distance is called displacement because it's measured from the center line it's not the total distance traveled it's just the distance from the center the maximum displacement so from the center to the crest or from the center to the TRU that is called the amplitude and it represents how much energy there is in a wave the greater the energy the higher the wave so very large wave like think tsunamis that destroy us all have a lot of energy and therefore very large amplitudes very small waves have very small amplitudes very small cute now what is a wavelength wavelength is the length of a wave but to be specific I want to talk about something short it's defined as the distance between two successive crests or two successive troughs so if I zoom into this transverse wave over here this is the wavelength or this is also the wavelength so from Crest to Crest or Tru to truff but a better understanding of what wavelength is is the fact that it's the distance traveled by a wave when it finishes one oscillation so let's talk about oscillations or vibrations for a second the feature of waves is vibrations molecules in a wave vibrate up and down they keep vibrating perpendicular to the wave motion if it's transvers to finish one complete oscillation you either start at the top which is a Crest go down and then back up that's one complete illation or you start at the bottom the molecule start at the bottom which is a trough go up and then back down that's one complete oscillation or if the wave starts at the Center I kind of spoiled it already if it goes up to the crest down to the truff then back to the center that's one complete oscillation so if I go back to the diagram down here what else can I consider one wavelength if you start at the center go up down and back to the center this distance horizontally is a wavelength so even though you need to Define it as the distance between two successive crests or two successive troughs I want you to also identify that it's also distance from the center you go up down back to the center that distance is the length of one wave okay good because that's one oscillation both amplitude and wavelength are distances essentially so they're measured in meters or centimeters depends let's move on to time so these two are units of distance meters let's go for time time or period in the case of a wave is the time taken for one complete oscillation how much time it takes you'll notice that there's a graph on the right side here and it also shows you that this is called one period time of one oscillation when is that a period and when is that going to be the wavelength when is that the period And when is that going to be the wavelength depends on the x-axis of the graph if the x-axis of the graph is distance it's wavelength if the x-axis is time it's period okay very good so we talked about period now let's talk about frequency which arguably is the most important part of a wave what is frequency quency since waves are all about vibrations and oscillations frequency is defined as the number of oscillations or Waves per unit time or per second this defines a lot of things about a wave for example in the case of light waves it affects the color of the light that you see in the case of sound it affects the pitch of the sound that you hear okay the unit of frequency is Hertz right the unit frequency is Hertz one Herz or 10 Herz for example means 10 oscillations or waves per second now when you want to calculate frequency you have two options either use the basic equation that uh is in your textbooks notes which is FAL 1/t keep in mind whenever we use a capital T we mean period not any total time no the time of one oscillation so if I tell you that the period is 2 seconds uh then the frequency is 1 over two which is half a Hertz so half an oscillation per second or something that we'll often use with multiple choice questions is if I give you a total number of waves if I say hey you have five waves that pass by in 10 seconds what is the frequency it's the same thing five waves in 10 seconds gives you 0.5 Hertz it's the same answer but the way the question gets phrased could be different so keep that in mind finally you have wave speed which is just speed distance over time but when we calculate the speed of a wave this is the formula we use V equal f * Lambda speed is equal to frequency times wavelength speed is equal to frequency times wavelength all right good very important let's move on now let's discuss the behavior of waves there are three General behaviors they work for like we said light or sound or anything so when we discuss these behaviors I'm going to be interchangeably using oh if you have a water wave if you have a light wave if you have a sound wave it's the same thing right because we're going to reapply this to sound and reapply this the light later on but for now what do I mean by reflection reflection is when a wave changes Direction so it's a change in direction of wave when it hits a surface so it has to strike something this could be a wall a mirror a human body it doesn't matter as long as a wave hits something it will change direction and reflect so if you have a light wave where we only draw the ray the wave hits the surface and changes Direction reflects cool now before I move on to drawing it with wavefronts and stuff there is one law the law of reflection states that the angle of incidence is equal to the angle of reflection or I equal R these angles are always measured from a line that we have to draw called the normal so we often draw a solid or dashed line that is perpendicular to the surface at 90° to the surface where the wave hits the surface so when you draw a line it hits the surface you stop right there draw a perpendicular line measure this angle which is the angle of incidence it's from the ray to the normal not the mirror or the surface the normal and it should be the same angle of reflection so this angle of instance if it were 60° or so the reflection would also be 60° what if you were to draw wave fronts alongside the uh the ray remember wave fronts are always drawn perpendicular to the ray they're always drawn at 90° to the ray so what we do is most of the time the question will show you like wave fronts already drawn and you have to draw the reflections so keep in mind the new reflected wave fronts have to be perpendicular to its Ray to their Rays okay this this one over here but trouble often ensues here right trouble often ensues here where you try to draw uh the W fronts so the problem is here as we were saying why because you need to complete like let me change the color of the reflected wave fronts for a second so you can see what I mean these are the reflected wavefronts so these are the actual wave fronts going to hit the wall and look at this they get smaller and smaller as more of them hits the wall and then you only have to draw the bit that gets reflected now this isn't too hard once you get used to it but keep in mind the amount that you draw and where you start drawing the wave fronts when the old wave fronts hit the surface that's if there are wave fronts involved finally sometimes diagrams use circular wavefronts this is what they look like this is ENT the same thing if this circle were to continue it would have looked like this roughly if this circle were to continue it would have looked like this so you draw their Reflections you draw their reflections over here also where the wave fronts touch the surface okay so that's reflection not too hard next refraction refraction is defined as the change in speed of a wave when the medium changes so if light travels from Air to glass that's a change in medium if sound travels from water to air that's a change in medium if water waves travel from deep water to shallow water that's a change in medium so something about the substance you're going through changes it could be an entirely different material or maybe even the temperature of the material changes like if by the way this is true if sound travels from hot air into cold air that's consider the change in medium the speed changes slightly because of the temperature of the air it changes its density okay so it's the change in speed when the medium changes noticed how I did not say change in Direction the primary thing here is the change in speed the direction could change but that's an extra thing that's an extra side effect it's not the primary thing when it comes to refraction so what is refraction change in speed when the medium changes if we're moving from a less dense to a more dense medium so figuratively if if we moving from a light to a denser medium the speed decreases and if the speed decreases according to V equals F Lambda if the speed decreases the wavelength decreases one very important thing the frequency never changes frequency is always constant nothing changes the frequency of a wave after it's been emitted nothing can change the frequency of a wave after it gets emitted so keep that in mind please so when we're moving from less Den to more Den the speed decreases and the wavelength decreases the wave can also Bend and it bends closer to the normal we'll see a couple of diagrams in a bit but if I'm moving from more dense to less dense so if I'm moving into a you know less dense medium to a nice empty medium Freer medium I move faster so the speed increases and the wavelength also increases the distance between the waves increases and the ray bends away from the normal again the frequency will never change keep that in mind okay now we just mentioned changing direction will the direction always change no it depends if the wave and specific specifically the ray if the ray of this wave hits the surface at 90° again let me zoom in for effect like look at this zoom in for effect if the wave hits the surface at 90° it doesn't bend it continues on straight no problem whatsoever it will only Bend if the ray hits the surface at a what at an angle so if now the ray hits the surface and then over here you can draw a normal which means you have an angle of incidence yeah now it's going to bend now how does it Bend you have two directions either the wave bends closer to the normal and this happens when you're moving from less to more dense and closer relative to where it was initially so it was going down it goes up a bit so the angle of refraction becomes smaller or if you're moving from more dense to less dense look at this the wave bends away from the normal the angle has to increase the angle of refraction compared to the angle of incidence increases it bends away from the normal now a hint for when we're solving diags and Ray diagrams here and wave fronts and everything if I ask you to sketch or to draw the wave fronts uh when when the wave bends my advice is you don't have to bother with the ray itself like just look at this and this I just want these two maybe these three okay look at these wave fronts grab your ruler extend it this way you're not drawing it yet this is your ruler and bend your your ruler closer to the surface closer to the surface the moment you bend the first one like the first wave front that's supposed to bend the rest are drawn parallel to it and if I want you to draw any more waves you continue with the same wavelength so if you're moving from less dense the more dense you bend the wave fronts closer to the surface to the boundary here it's the opposite you bend your wave fronts away from the boundary and the rest are parallel to it if you've drawn things correctly the wavelength should increase properly okay so that's refraction the final behavior is called defraction and it is defined as the spreading of a wave as it passes through a gap or around the corner or an edge what do I mean if you have a gap like this essentially a wall with a hole in it or something and the wave goes through it spreads a little bit it stays straight but it spreads spreads a little bit like the edges curve a little but it doesn't spread much by the way like just a bit but that depends on what the size of the gap Gap compared to the wavelength if the Gap is very large compared to the wavelength it doesn't spread much it's very little defraction and it doesn't curve it's just straight but if you make the Gap smaller if you make the Gap smaller and again smaller or larger is relative to the wavelength so depending on the size of the wavelength so if the Gap is smaller so if the gap size decreases or if if you increase the wavelength that also works it spreads more and the entire shape of the wave changes it curves now one mistake a lot of people make when answering questions or sketching this diagram is accidentally not drawing the same wavelength meaning if you see this wavelength over here try to at least by I try to keep the wavelength the same try to keep the wavelength the same all right that's very important okay so again a wave front is nothing more because we have a question here a wave front is nothing more than this line over here this is a Crest it essentially shows you what is a Crest right so if you draw a wave front going through a gap it doesn't spread much but if it goes through a small Gap it curves a lot curves a lot okay but it doesn't always work with gaps sometimes it works with edges what do I mean by edges what if you don't have a a gap you just have the corner of a wall or a corner of a building okay if the wavelength because now you only have wavelength to compare to if the wavelength is very small cuz there's no Gap it doesn't spread much spreads very little so you draw these a straight but then the edges are a little bit curved something you can do is draw this dotted line for reference like if you see a small wavelength just draw a dotted line to show you that you're not going to spread much so when you draw these lines you can stop at them you can stop at them and just curve the edge and then and then if the wavelength is very large wavelength is very large the Gap hasn't changed there is no Gap it's an edge it curves a lot more now I want you to draw it a straight line almost almost until you reach the edge of the wall like this isn't perfect either but almost until you reach the edge of the wall and then the rest of it will what will curve please wait I'll answer your questions right away okay good so if the wavelength is large and you just have an edge you curve it a lot if the wavelength is small it doesn't curve a lot now one of the questions that I remember in the past papers that used this concept was something like a mountain like hey you have a mountain and you have uh radio signals and you know microwave signals that are being released by some kind of radio tower and you have waves and you have a car on the other side and the question was literally hey why do the radio waves reach the car on the other side of the mountain but the microwaves which have a smaller wavelength why would the microwaves not reach the car H because the micay have a smaller what wavelength so they don't curve much they don't defract much but because the radio waves have a larger wavelength they curve a lot more and therefore the radio waves reach the car so your phone signal won't get to you CU phone signals are microwaves but the radio signal of your radio will get to you if you're past the mountain and all of them are being released by the same Tower cap Kut very good so let's solve a few questions about the general wave properties here first question asks you two types of seismic waves are p waves and S waves so remember p waves are primary they're underground longitudinal S waves are on the surface and they're Trans State the types of waves that p waves and S waves can be modeled as so we set p waves areal and S waves are next the velocity of a p wve in the earth's solid crust is 7.2 look at this look at the units 7.2 km/s and the frequency is 4.5 Herz what is the wavelength of this P wve now the formula let's change the spect to Red the formula is v = f * Lambda where f is frequency Lambda is wavelength and V is speed or velocity so Lambda is equal to V / f now hold on hold on I know some of you have you know noticed but if I just substitute 7.2 / 4.5 and I pull out my calculator and I start typing these numbers and some people are already screaming and shat Mr the units this is going to be 1.6 I can leave it like that if I've noticed the unit and write it down as kilometers or you change it to meters like you've been trained to like hey I like everything to be SI units so that's 6, 1,600 M both of these answers are correct just make sure the unit supports the answer you've written down so I'm going to write it down in SI units because I'm used to that but 1.6 kilm is fine okay that's why it's three marks one Mark is for that unit difference a change in unit next a transverse wave moves along a rope the diagram shows the position of the Rope at one particular time which two labeled points are one wavelength apart so remember Crest to Crest or Tru to Tru and ooh he didn't give me Crest to Crest or Tru to truff Sure W's at the crest but I'm not giving any other crests so remember what we said if you start in the center so if I start in the center I have to go down up and then back to the center so I have to touch both a Crest and a truff again I have to touch both the crest and the truff so the distance is X what not x what y it's x and z so C is my correct answer CU that's the wavelength okay good very good next why not W and X really this one over here this a little bit over here this little bit or are you trolling you're trolling aren't you this is a this isn't even half a wave it's a quarter of a wave so it can't even be a wavelength so nope C now never mind let's move on the diagrams show graphs of displacement Against Time for four waves all the graphs are drawn to scale which wave has the largest amplitude and the highest frequency so largest amplitude is easy it's a or b because they're both you know two divisions high so that's the largest you largest frequency is the largest number of waves per second so obviously that's a because I see more waves if we count it's 1 2 3 four five and six waves compared to the one and not even two waves like one and 3/4 next figure 6.1 shows the crests of a plain water wave approaching a barrier in a gap draw three crests of the water wave to the right of the barrier now you need to ask yourself is the gap small or large you don't need to think too hard small so what do you do going to draw it curved and spreading out a lot one thing you can do is to draw these remember those supporting dots so like it's going to curve a lot so this is where it's going to stop maybe if it hits the wall that's fine but let me show you the wrong way of doing this you're going to go like ha see there we go can you tell me why this is wrong like I've curved it right but why is this wrong H why is this wrong because of the wavelength the wavelength is not the same the wavelength shouldn't change so if I were you I would at least at least measure oops sorry about that the application really didn't like me drawing erasing that now I would at least measure what the wavelength here and try to keep it roughly the same try to does it have to be 100% accurate No but at least by I it should look good like if this is after one wavelength and this is after a second and this is after a third now I draw what the curves CU which graph is this Which diagram sorry is this the small Gap or the large gap it's clearly the small Gap right it curves a lot huh remember we mentioned this earlier it curves a it'll curve a lot but the problem is you need to keep the wavelength the same this is is the wavelength so this should be the same the distance between your crests or wavefront should be the same then the question changes the the topic it's not the fraction anymore it's what refraction now figure 6.2 shows the crests of plain water waves in deep water approaching a region of shallow water now now the question is already spoiled build it for us but you should have already memorized that yourself water moves slowly in shallow water waves move slowly in shallow water but fast in deep water just memorize that for now deep water means they move fast shallow means they move slow so the shallow is the more dense medium huh what does he want me to do on figure 6.2 draw prests of the water wave in the shallow water huh he didn't even call them wave fronts he called them CR it's the same thing and the direction of travel of the water in the shallow water this means the wave fronts whereas this means you need to draw the ray so you know how many how many does he want he said three crests okay let's start with the crest I know we should start with the ray but let's start with the crests first this is the wave right like if there was no boundary if there was no medium straight because it's in the more dense medium it'll Bend closer to the normal so just bend it so it's closer don't bend it too close like don't touch the surface just bend it a little bit so that you can visually see I don't care if you make it a bit too short a bit too long that's fine longer is a bit better and then from the next wave front like this one that's touching the surface draw the same same wave front with the same angle always start by continuing the wave fronts that have already touched the boundary but sir he wants a third one okay draw one more right after that with the same angle and with the same wavefront wavelength roughly it's not going to be perfect but roughly the same see I'm going to move it up just a bit there we go those are three absolutely uh let's count one two three three reference right does it matter if it's short or long or I'm a short human being and do not do I not count as one human being or but these are new ones after they've been okay now hey wait wait a second where's the ray well first continue this Ray when it hits the surface bend the ray so it's perpendicular to the new wave fronts that you've drawn you must draw an arrow you must draw an arrow first it has to be perpendicular to these wavefronts and you must draw an arrow without it it doesn't count so here are the crests here's the direction of motion moving on state two ways in which transverse waves differ from longitudinal waves H state two ways in which transverse wav differ from longitudinal wave do you remember what they are H let's go back what's a transverse wave what's the longitudal wave they're right here they're right here so state two waves in which they differ transverse waves are waves where the molecules vibrate perpendicular to wave motion longitudinal waves are waves where the molecules vibrate parallel to wave motion cool so that's one answer what's the other what do you think is the other you can just mention the what they're made of a transverse wave consists of what crests and troughs long theune wave consists of what compressions and ractions these are two different Answers by the way cuz one you talk about the molecules vibration like how do molecules vibrate and move and the second is what does it consist of so uh particles and transverse waves vibrate perpendicular to wave motion right and particle or sorry transverse waves consist of rest and maybe I should make the font a bit smaller so it could fit but you know this is fine finally a radio station broadcasts a signal with a frequency of 89 what's capital m h what's capital M answer right away type it type it what's capital M M Mega bravo bravo mega mega is similar to Million by the way so it's times a thousand, or come on we're better at standard form here 10 the^ of six so that's 89 10 the^ of 6 what is the wavelength of this signal let's work it out V equal f * Lambda so Lambda equal v/ f sir I don't have the V I remember like putting this question in or like a fair having a similar question to this in a mock exam once and everybody was crying like they didn't know the speed we're going to study this we're going to revise this today but since he said radio waves all electromagnetic waves have the same speed which is the speed of light that speed that speed is 3 * 10 ^ of 8 m/s so the reason why the speed wasn't given was because you were supposed to memorize that value of speed it applies to radi waves x-rays microwaves ultraviolet everything over 89 * 10 ^ of 6 let's type that down 3 10^ 8 over 89 10^ 6 this gives me 3.37 m so the correct answer here is 3.4 because that's rounded up Noy doie remember examiners can be evil by the way because if you forget to change ah Mega means times a million times 10 the^ of six so if you forget to change the mega and just leave it as 89 you will get uh 33707 change that to standard form it's 3.37 10 ^ of 6 it'll give you 3.4 Mega so keep that in mind okay how did I get 3 * 10 the^ of 8 you memorize it like I said you have to memorize the speed of light or that's the speed of all electromagnetic waves okay very good next next now we're done with most of the wave properties and here's the thing before I continue if you properly understand this part of the unit the rest of it becomes very easy like discussing sound waves and electromagnetic waves and light waves comes very easy because it's just all of this info again just we focus on slightly different details that are exclusive to each specific wave so for example now we're going to talk about sound waves right you'll notice that we're going to describe what a sound wave is like it's mechanical longitude we're going to talk about the speed of sound and what medium effects it we're going to talk about amplitude and frequency we'll talk about the reflection of sound so as long as you understand the basic wave properties and Concepts here the rest is simple now let's talk about sound waves first what type of a wave is a sound wave it's a mechanical longitudinal wave meaning it needs a medium to go through and the molecules vibrate parallel to wave motion it consists of compressions and R fractions but one extra special thing about sound waves is that when sound waves travel through air creating its compressions and rare factions and compressions and rare factions during a compression the molecules are actually closer together than usual which causes the pressure of the air to increase because the molecules are colliding more frequently with each other but during a rare faction the particles are farther away from each other so they Collide less frequently so the pressure of the air decreases so if I were to draw a graph of how the pressure of the air changes over like the distance or over time with this not being zero here's the zero with this being normal atmospheric pressure I'll just say it's 1 * 10 the^ of 5 like it has a value you don't memorize that though it looked like this it looks like a transverse wave but this is a graph keep that in mind when the pressure is really high that's a compression when the pressure is really low that's a rare fraction all right so keep in mind sound waves do change the pressure of the air it's going through okay next let's talk about the speed of sound refraction is the change in speed when the medium changes which means when sound travels from one medium to another the speed of sound changes you need to memorize three values for the speed of sound speed of sound in air is is 330 m/s it's got a range 330 to 350 but 330 is fine in water the speed is 1,500 m/s and in steel solids essentially it's about 5,000 or 6,000 m/ second which means sound travels the fastest in solids but the slowest in gases why because sound waves are uh longitudinal the molecules vibrate parallel to the direction of wave motion since solid molecules are already super close to each other it's very easy for it to transmit sound quickly and since gas molecules are all far apart relying on them to transfer sound just vibrations forward and backwards is quite slow compared to solids 330 m/ second is still really fast for us human being set now how do I find the speed of sound now if you ever study as physics we have a much more accurate experiment to find the speed of sound so far more accurate than this nonsense that we're going to describe but this is very nice and basic you can do this at home grab a brother or a friend or kidnap somebody and then give them a stopwatch make sure they don't move from their place so maybe have them sit down in a chair all right maybe tie them to the chair and you get a gun now obviously this gun does not shoot bullets okay maybe it does this gun does not shoot bullets but this gun produces two things a large puff of smoke or a flash so you can see it and sound here's the question what travels faster the light or the you get steps on how to con someone no this tutorial is about physics okay not about kidnapping people cool light obviously travels faster and light travels so fast it's almost negligible the time it takes to travel is almost negligible and then you hear the sound so essentially the way this works is put your you know uh assistant over here with a stopwatch you stand far away the larger the distance the better so at least 100 m all right but even more is better so at 300 M that's a lot better because now the time will be about a second which is good so stand about 100 to 300 M away and then when you fire the smoke pistol your assistant will see the puff of smoke first or the flash first and then after a short time they will hear the sound so they press start when they see the flash or the smoke and they press stop when they hear the sound and now you have the time you measure the distance with a measuring tape you measure the time with that softw watch that's with your you know uh Force assistant and then speed is distance over time repeat and take outage okay next how does amplitude and how does frequency affect the sound that you hear amplitude affects the loudness the larger the amplitude the larger the loudness of the sound the sound is much larger louder right the lower the amplitude the lower the loudness of the cell then frequency affects the pitch the higher the frequency because look look at this look at this this is a high frequency wave this is a low frequency wave high pitches sound very uh very sharp like I'm going to change my pitch for a bit but it does hurt my throat so I hope it doesn't hurt me the rest of the day if I raise the frequency of my voice that's me increasing my pitch but if I lower my pitch the frequency decreases so frequency affects the pitch of the sound that you hear all right frequency affects the pitch of the sound that you hear very good now we don't hear all frequencies though we as human beings hear from 20 HZ to 20,000 Herz like from 20 Herz the smallest frequency to 20,000 Hertz that's the highest frequency that we can hear anything less than 20 is called an infrasound the word infra means smaller than anything greater than 20,000 is called an ultrasound all right now what do we use ultrasound for now for art obviously because look at this fantastic but besides that we use ultrasounds for three things primarily sonar which is a way of detecting objects underwater so if you have a ship right and you want to know if you've got any fishes you know any schools of fish down here or whales or submarines or Torpedoes coming in at you from some kind of enemy doesn't matter you emit ultrasounds underwater when it hits something it reflects and when we detect it we can see what's or at least know how far it is and how large the thing is it's underwater it's called sonar Medical Imaging or ultrasound is used to see what's inside a human being like pregnant ladies or uh for example I had an ultrasound dawn on me again it's not because I was pregnant but I'm sorry can't keep it straight face but because uh they were checking uh like my liver and stuff so the way they do it is they take that ultrasound transducer the word transducer is a device that produces and detects both produces and detects ultrasound so emits and receives so it only two different devices if you forget the word transducer in your description just say uh you get a ultrasound producer and receiver emitter and receiver that's fine they take that put it on your you know belly they put that really cold annoying gel it's important because without it The Sound doesn't go through your skin you put that you know transducer on your belly and they move it around and whenever the sound hits something inside like hits a bone hits an organ hits a baby it reflects back and then we use that signal to see what the baby looks like all right yeah how does the gel let the sound go through your stomach this is out beyond the scope of the syllabus but uh if the sound waves hit the surface like from the detector to the surface of the skin because it's moving from Air to your body your skin and fat and muscle that large difference in medium causes most of the sound to reflect I don't want it to reflect when it hits your skin I want it to reflect when it goes through and hits something else so when you put that gel that gel covers up the detector and emitter the transducer so when it transmits the sound it doesn't see the skin or say it thinks oh everything's the same because the gel has the same density as for the sound the same density as your skin so just goes through without it The Sound would actually reflect most of it would reflect off of my skin and it wouldn't go through now the last thing we do or we use ultrasound for is for the non-destructive testing of materials what do I mean by that we use it to see if there are any gaps or cracks inside something solid so we put our transducer and let's say this piece of metal was manufactured with a little bubble inside or a crack inside but I don't want to break the whole thing to see if there's any cracks inside so we allow the ultrasounds to go through this object if it's going through a full solid object nothing happens it reflects only at the very end of the metal but if there's a crack it's going to reflect early so whenever the ultrasound hits the crack or the bubble it reflects not just once but twice because it reflects from the first surface reflects from the second surface so when we see the signal on our uh you know oscilloscope we'll have the initial pulse this is what we emitted this is the first reflection this is the second reflection this shows us the time it took to go through the bubble we essentially use the difference in time to get the size of the bubble and we use the time taken for it to reflect first to get the distance it traveled before it hits the bubble so how deep the bubble is that's what we mean by non-destructive testing material since we've used the term reflection a lot here the reflection of sound is called a what an echo it's just the reflection of sound one thing you have to note though when we're solving any questions about Echoes is that the time taken to hear an echo is the time taken for it to let's say you shout you shout like pizza and then Pizza goes hits the cliff and comes back pizza now you hear and let's say that took 1.3 seconds that's a lot by the way but let's say it took 1.3 seconds this is not the time taken for the sound to reflect this is the time taken for the sound to go to the cliff and back so if I give you the distance or sorry if I give you the speed of sound is 3 30 m/s the distance that you get is the distance to the wall and back not just to the wall so if I want the distance to the wall this is what I need to do oh distance is speed times time so the speed is 330 * 1.3 IDE 2 CU 1.3 seconds is the time taken for it to go to and back which is why here in the notes I've written down as speed is double the distance over time it doesn't mean the distance is actually double but the D here is not the distance traveled by the sound this is the distance to the wall or to the reflecting surface right this is shown by the very first question we're solving right now look at this let's solve this a boy shouts and here's the echo from a tall building 2.2 seconds later so you hear the echo after 2.2 seconds the speed of sound is 330 how far away is the boy from the building distance equals speed time time the speed is 330 * 2. no this will give you the distance to the building in back I don't want the distance to the building back I want just half of that I just want it to the building so you divide that by two so 330 * 2.2 over 2 gives you 3.63 which is 360 good let's solve another one sound waves have compressions and rare fractions what is meant by a compression it is a region where the particles are close to each other now if the question hasn't specified I can use pressure in my description it's a region where the pressure region the air where the pressure is greater than normal atmospheric a rare fraction is a region where the particles are far apart or where the pressure is lower than normal atmosphere keep that in mind sometimes the questions description questions like these will specify explain what is meant by compressions and refractions in terms of particles don't mention pressure in terms of pressure don't mention particles okay uh we can see the light can I write things in Brackets no you can't the brackets are there to show you what alternate answers could be do not give an examiner a choice of answers cuz if one of them is wrong the entire answer is wrong keep that in mind so when mark schemes use brackets as well or slashes or dashes they're showing you other alternate accepted answers not hey you should copy this and do that as well okay we can see light from the Sun but we cannot hear any sound from it why because sound cannot travel through a vacuum because sound is mechanical can't go through space I know some of you thought light is faster than sound I know some of you did you wrote that but no it's because by the way by the way the sun if you think about it does produce sound sun is a big nuclear fusion reaction so it's probably lots of explosions happening in there but we don't hear it thank God we don't cuz sound cannot travel through empty space the sun would be very loud during a thunderstorm an observer sees the lightning almost immediately but hears the sound of the Thunder several seconds later the thunder and lightning are produced at the same time obviously because lightning is faster than thunder light is faster than sound here it's faster the sound of the Thunder is heard 9 seconds after seeing the lightning the speed of sound is 340 m/ second what is the distance to the Observer this is not an echo question this is a very basic distance equals speed times time question this is not an echo question this is very basic 340 * Ste question okay nothing special next A student makes a list of some applications of waves medical scanning of soft tissue sterilizing water using Sonar to calculate the depths of the ocean which of course which applications use ultrasound now medical scanning of soft tissue yep sterilizing water no this is actually this is actually Ultra the use for ultraviolet radiation sound doesn't sterilize ultraviolet radiation does we'll mention that right now like we're heading down to the electromagnetic spectrum to finish off the class sonar yep so one and what three so our answer is same okay uh do you have a question in the previous uh question no okay okay so let's finish up this part of the unit not this unit this part of the unit by talking about the electromagnetic spectrum what is the electromagnetic spectrum it's all of the electromagnetic waves that exist in reality in the universe at least as far as we know but before we discuss the differences and what their names are what you use them for ETC few common points all of the electromagnetic waves that we will discuss can all travel through vacuum that's a feature they all have the same speed in air or a vacuum which is 3 * 10 ^ of 8 m/s you need to memorize this you need to memorize this it's not going to be given sometimes it will be given in questions but not always and then all of them are transverse there's no such thing as a longitudinal electromagnetic wave they're all transfers okay so how do you memorize the Spectrum and why are all these waves ordered this way they're ordered in the order of their frequency where as you go to the right the frequency increases so GMA Rays have the highest frequency radio waves have the lowest frequency but keep in mind that frequency and wavelength are inversely proportional if I have a very high frequency wave naturally the wavelength is smaller if you have a low frequency wave so you have less oscillations per unit time naturally the wavelength is Lar so gar have the highest frequency the shortest wavelength radio waves have the lowest frequency but the longest wavelength so how do you memorize these then my advice is always start in the Middle with visible light and remember that visible light consists of seven different colors red orange yellow green blue indigo and violet Roy GI Roy GB right so Mr Roy middle name like middle letter G Biv Mr Biv anything higher than violet is ultra violet because the word Ultra means greater than right wish there was a plus ultraviolet then would have been cool now if anybody gets that reference here cool now xrays are higher than ultraviolet and GMA Rays have a higher frequency than x-rays these aren't like one specific values ultraviolet waves have a range of values yes xrays have a range of values gamma rays have a range of values so keep that in mind it's like a group of values if you go lower than red you have infra red so less than red then if you go less than red you have microwaves and then you have radio waves now most of the waves on the right side these high frequency waves are harmful they are are ionizing they can cause damage and the higher frequency they are the more harmful they are so GMA rays are the most harmful x-rays slightly less so ultraviolet harmful but not so much infrared microwaves and radi waves in essence are not very harmful but they do have their own dangers we'll discuss them in a bit not radio though that that's literally nothing but first what do we use them for starting from the bottom radio waves are obviously used for commun iation right radio and TV Transmissions or what we sometimes like to call terrestrial communication meaning on Earth we don't send it to satellites and back we don't use that we don't use radio for that we also use it for Bluetooth communication okay then we use it sometimes for astronomy when we send radi waves out into the universe not to satellites into the universe or try to see if we can detect radio waves right uh RFID you all know what RFID is you know those tags that you use uh I I use it for my elevator here because it's got an RFID tag I have an RFID tag for my for school when you I log in and open up you know security doors so microwaves they're used for satellite television mobile phones and Wi-Fi these are the primary uses I know everybody thinks of uh ovens but that's not the primary use the primary use is satellite and mobile phones only one specific value is used in microwave ovens and that value is the natural frequency of water so it doesn't actually heat your food directly it vibrates the water molecules inside which heats up the rest of your food this is where the danger comes from not these these aren't dangerous these are infrared we talked about them in unit two so they transfer heat energy so we use them for grills and heating and cooking but we also use them for short range communication like remote controls Intruder alarms you know those uh you know those elevators that close and open or like M doors that open and close nor like uh automatically that's because they have an infrared emitter at the top that emits infrared waves and there's a reflector on the ground when you break that beam you don't see it it's literally a laser beam it's invisible when you break that beam it opens the door for you Intruder alarms work the same way but we put them on Windows and like doors when we close off a shop these turn on so on thermal imaging you know pictures that show you which spots are hot and which spots are cold optic fibers these are fibers that transmit light beams or infrared beams and they're used for internet communication or medic uses we'll discuss them next time visible light uh what do we use that for we use it to see Bravo and take pictures ultraviolet we use it to sterilize water because we saw that question earlier and we use it for security markings and detecting Bank notes if they're fake so for example if I have a bank note over here oh oh no oh no I dropped my money my money my money I had my money right next to me just if I have a bank note here and I put it under regular light you might see some kind of marking but if you put it under sunlight you'll see a completely different thing you'll see like a hidden either number or a hidden Mark somewhere a hidden picture or another hidden line like usually there are lines in here that you don't see unless you expose the bills to ultraviolet radiation x-rays are used for medical scanning to check for broken bones because x-rays can go through your flesh but not your bones the more dense something is extra can go through them security scans they check what's inside your bag so if I have uh I don't know my tablet inside my bag my iPad my gun my knife my wallet a few of those things are odd like my wallet but still the more dense something is inside it is stop the x-rays stop when they hit them so the machine sees them right very good gamma radiation is used to sterilize medical equipment and food so if you have cans you expose them to Gar Rays it kills off any kind of bacteria in there if you have medical equipment like you know scapel and forceps that you know doctors use during surgery they're exposed to Gar Rays after they're cleaned obviously in order to kill off any kind of bacteria or viruses on them if there's any that we don't see and we can also use it to detect cancer and treat it but we'll discuss that in unit five in detail okay next up we mentioned that they're harmful specifically what microwaves are harmful that single frequency that you use in the oven because it heats up your cells on the inside infrared can burn you if it's you know hot obviously ultraviolet can damage your eyes and cells or can give you skin cancer if you're exposed to too much sunlight if the sunlight is too strong it's not just the heat that gets you it's the ultraviolet radiation that can get you xrays and gar rays are both ionizing so they can damage your cells or mutate them now satellites I'll be honest with you about something this has not been used in any questions yet like the types of satellites that we use microwaves for we use microwaves for satellite communication have not shown up in questions yet it's in the syllabus you'll find a line or two about them somewhere here hold on oh yeah there we go look at this this is from the syllabus know that communication with artificial satellites is mainly by microwaves some satellite phones use low orbits artificial satellites some satellite phones and broadcast TV use geostationary satellites so what's the difference low orbit satellites are satellites that just orbit the earth in any angle or direction we choose we get to choose where it goes it's mostly used for phones for satellite bones but geostationary orbits are satellites that orbit the Earth around the Equator so it has to be parallel to the equator and on and it also like orbits from west to east but the important thing is it's always above the equator and it takes 24 hours to orbit meaning it rotates with the Earth which is why we use it for TV so we send microwaves like let's say I live here in Egypt we send microwaves to the satellite the satellite sends out that same wave to the entire region so everybody watches the same channels and because it's always in the same place we get the same signal it's called a geostationary orbit the word geo means Earth and stationary means not moving you get the DI idea then the starting here are things that have been added new to the syllabus in 2023 what do I mean any exams before 2023 you will never see any of these questions yet okay so for example what do mobile phones use they use microwaves and Wi-Fi they use microwaves why you have to memorize the advantage because microwaves can penetrate through walls and only need a short aiel or a short antenna for transmission reception these are things that you have to unfortunately just memorize and state what do we use Bluetooth for or like what type of wave do you use Bluetooth or radio waves why because radio waves can pass through walls that's the advantage disadvantage the signal gets weaker when you when they pass through walls what do we use optic fibers for or what types of ways do you use in optic fibers visible light and threed so highspeed communication Broadband means internet to those of you who don't know the word Broadband highspeed Broadband means internet okay and why do we use them because number one it's the first Advantage optic fibers are transparent to visible light and infrared which means they can undergo total internal reflection inside some other waves like GMA Rays don't even care that you know like glass exists or Optic fibers exist just go through and second infrared waves invisible light that's the second Advantage can carry High rates of data meaning they can transmit information very quickly IR rate means very quickly next when it comes to sending signals through optic fibers or through you know microwaves or through radio waves we have two general signals digital and analog digital signals have either one of two values high or low this is what computers use you know ones and zeros 1 0 1 0 1 1 1 0 0 0 one zero and analog signals are like my the sound of my voice my voice is analog meaning I can have any I'm sorry I know that sounded funny but I wanted to prove a point I it can change to any value within a specific range so anlog signals are more natural signals you know the music that you hear the wind flowing you know an ostd that I like to listen to when I work like all of these are analog signals digital signals that go through computers or the internet or everything else are basically just computer code high or low one or zero there's a current there's no current there's light there's no light and digital is what we always use when we're transmitting signals why number one transmits information very fast has a high rate of data transmission and second you can transmit it for a longer distance because think of it this way digital signals are ones and zeros optic fibers use digital signals which use light INF for red basically you grabbing a laser pointer and just like pressing the button on off on on and off on and off on and off and light travels very quickly at 3 * 10^ 8 so you can use optic fibers which use visible light to transmit digital signals and second over time any kind of signal over time any signal like let's say this is a signal gets weaker and weaker and weaker this is bad because this means there's a limit to how far we can a transmit it digital signals do get weak but do you know what the advantage of digital is compared to analog is that I only have two values so when we put a device called an amplifier anybody who plays guitar probably have has heard of this before but an amplifier is just a device that makes the signal stronger again regenerates it an amplifier makes the signal strong again since it's digital we know it's either one or zero so cool but analog signals when they get weaker become very hard because that means we can mess up the signal when we amplify it because we don't know what the original values were that's a lot of by the way rant for something as simple as the advantage of digital over analog you just need to memorize these two and why because digital signals aren't affected by noise this can honestly be considered the third Advantage if you want to if I ask you to sketch a graph showing the difference between the signals this is a good sketch for digital this is a good sketch for anlock just draw a wave like if you just draw a regular wave for analog that's fine signal is just like high zero low one zero that's it so let's finish this up by solving a couple of questions a mobile phone network uses microwaves of frequency 9 1.9 10 the^ of 9 Herz to transmit and receive signals the speed of micro is 3 * 10^ 8 m/s what is the wavelength well that's easy v = f * Lambda so Lambda = v/ F there's no trick here honestly if I were the examiner I would have probably tried to trick you and not give you this but you've already seen that you know trick before uh 3 * 10^ 8 over 1.9 10^ 9 this gives me 0578 1.58 is good enough sorry oh .158 is good enough met state two reasons why microwaves are used for mobile phone signals number one they can pass through some walls and number two they only need a short aerial or antenna to send and receive next all mobile phone networks use digital signals communicate with the phone describe with the aid of a diagram not you may draw a diagram no no no no no with the aid of a diagram meaning you have to draw the diagram how a digital signal differs from an analog signal so let's describe first digital signals consist of one of two values high or low analog sign can have any value within and how do I sketch them sketch the two graphs I showed you earlier this is digital this is analog I don't care where the x-axis is that's fine state two advantages of using digital signals rather than analog so digital signals uh just the advantes so uh they transmit dat faster so oh if you forget the word rate of data transmission or greater rate of data transmission that's fine uh they have a greater range you can also say they're less affected by noise so that's the first question second two types of electromagnetic radiation are used in glass Optical fibers for highspeed Broadband State the type of electromagnetic radiation other than visible light which which is used in glass optic fibers so other than visible light we said we have infrared waves then give two reasons why these two types of electromagnetic radiations are used in glass optic fibers for high speed Broadband huh remember I told you you're going to memorize these so if you scroll back up huh why number one the transparent visible light glass is transparent to visible light and number two they can carry High rates of data that's it copy the same answers class is transparent to visible like and in and then they can transmit High rates of data or they transmit the information quickly or they transmit data quickly good I think this is the last one a television TV station transmits a signal to a television receiving dish the television has an on andof indicator the television is switched on by a remote control oops I wanted to zoom in by a remote control which changes the indicator light from red to Green which electromagnetic wave has the longest wavelength ooh hold on hold on let's write down the names of these waves uh from a TV station to a satellite these are microwaves then from the remote control to the DV that's infrared and red and green are both visible light with red having a longer wavelength than green so which one has the longest wavelength so if you remember the Spectrum longest is radio and then micro and then infrared so a is your answer because if I write them down in order from longest to shortest you have Micro then infrared then red then green very nice yeah that's the end of it and that ladies and gentlemen is the end of the first half of unit three where we discussed the properties of waves sound waves and the electromagnetic spectrum