B Tech applied science this is for unit one the physics and this video is about refraction and TI total internal reflection so here we go now A little quiz to start with what are these waves doing in each diagram what are they doing so have a little think you should know that it is so the first one reflection not particularly bothered about that uh defraction later on in this unit that's important when we do defraction gratings and things defraction this is what I'm interested in now which is refraction refraction there is a wave changing direction as it goes through this glass block and that is refraction okay uh when a wave enters a denser medium so a wave travels through a medium if it goes into a denser medium for example if it goes from Air to Glass then it bends towards the normal so this dotted line here is the normal the word normal just means at right angles and you can see that this wave here bends towards the normal so when it gets denser it's towards the normal why does this happen if you just remember it's because the wave slows down down yes like a car driving into a muddy field it slows down and that's all you need to know I'm not going to explain it any more than that because you won't have to so when it goes into a denser medium it goes towards the normal because the wave slows down now when it goes from glass to air that's going to a less dense medium so it bends away from the normal because the wave speeds up so the wave is in glass and it goes into air and it bends away from the normal why does it do that because it speeds up okay so remember them now the speed of light in a in a vacuum you will know is 3 * 10 8 m/s uh the speed of light in water is just 2.25 * 10 8 so it's less okay because water is denser we say it's more optically dense than air now the refractive index is how much it slows down so n which is always bigger than one by the way remember n is always bigger than one now the refractive index of water is 1.33 now how did I get that because it's the speed of light in air or in a vacuum which is C yes 3 * 10 8 divided by the speed of light in water which is v in this equation so n the refractive index is basically if you like it's how much faster it travels in a vacuum yeah it's the ratio of the speed of light in a vacuum to the speed of light in a medium so it's the property of the medium it's the property of water the refractive index of water is 1.33 and this is our first equation there's a few equations we need to know n = c over V do you reckon you could do this question here pause the video pen paper calculator and I'll show you the answer in 3 2 1 okay so Nal C over V so let's rearrange the equation so v = c / n so 3 * 10 8 so divided 2.4 is 1.25 * 10 8 it's a very very dense material Diamond very very optically dense material now there's another equation that we need to know which is a little bit trickier we've got Nal C over V now n is also Sin i/ Sin R now what are ion now we look at the diagram i and r are the two angles and I is the bigger angle if you think about it I told you before that n has to be bigger than one so sin I has to be bigger than sin R for n to be bigger than one uh and so I has to be bigger than R so strictly speaking it I is the angle of incidence and R is the angle of refraction but you know which way the light's going doesn't matter whether it's entering the glass or leav the glass I is the bigger angle R is the smaller angle okay uh remember also that both angles are measured between the Ray and the normal so it's the angle between the red line and the dotted line yes between the Ray and the normal if you're very lucky you'll be given both angles and then on your calculator you can do sin I and sin R and then just divide one by the other to work out n so if you're lucky they might ask you to work out n uh if not you might get a smelly question like this one here where you have to work out the angle of incidence so it's giv you the refractive index it's giving you the angle of refraction and you have to work out the angle of incidence if you don't do maths then I understand this is tricky have a go at it I'll show you the answer in three 2 one okay so we want to work out the angle of incidence so what we can do is we can work out s i so s i if you bring that up there so sin I is n * sin R and then I will be sin to theus1 of n * sin R then put in the numbers 41.7 okay uh as I said I understand that's a bit mathsy a bit tricky but there you go I'm here to teach physics not trigonometry so there you go that's how you do it so look at this diagram here's my ray of light and it's traveling in water maybe I'm holding a laser underwater and it's leaving the water and it's going into H so that's less dense so it's bending away from the normal okay now let's make that angle there let's make that bigger and let's see what happens so notice I'm making this angle in the water I'm making that bigger and so the angle outside the water see this angle here that gets bigger as well okay but it can't keep getting bigger and can you see why because the biggest that that angle can be is 90° so if I'm making this angle here bigger and bigger and bigger at some point uh the angle leaving the water it can only refract 90° so what will happen if I make this angle even bigger well if it can't refract then what it will do is it will reflect so when we reach a certain critical angle when this angle here reaches a critical angle when it's big enough when it reaches a certain value what happens is that the wave can't refract so it reflects so the surface of the water will act like a mirror you might have noticed it if ever you've been underwater and you've looked up at the surface of the water the surface of the water will act like a mirror and this is called total internal reflection reflection because it's reflecting internal because it's inside the water and total because all of the light is reflecting so total internal reflection yeah the wave can't refract more than 90° so it reflects here's a fantastic animation that I've done and if you watch that for a few times and you will notice okay so at the moment the wave is refracting yeah it's refracting but when we get to an angle about here yeah this angle here then the wave starts to reflect yes missed it a little bit but you get the idea so when this angle reaches a certain value you the wave starts to reflect and that is called the critical angle and that's when total internal reflection happens now I've called the critical angle Capital C uh because that's what we call it at a level I mean little C is the speed of light and Big C is the critical angle and here's our last equation I promise sin c = 1 / n so if you know the critical angle you can work out the refractive index if you know the refractive index you can work out the critical angle so your next job is to work out them work out the critical angle for water and for glass and for Diamond so pause the video have a go yourself and the answers are there you go 48.8 41.8 24 .6 you can't go wrong with three sigfigs notice the critical angle for diamond is quite small so rays of light kind of get trapped inside a diamond and then they all come out at the same angle which is why Diamond Sparkle now is this total internal reflection is it any use well it is because look at this diagram this is a glass fiber which is is a a very thin wire made out of glass a fiber made out of glass and if we put light in one end of the fiber it can bounce around and come out of the other end and the point is that light travels in straight lines well we can actually get it going around corners and getting light to go wherever we want if we can trap it inside these glass fibers and how does it stay inside the fiber because it reflects off the inside of the fiber by total internal reflection okay so on all of these diagrams here yes uh this angle here is always bigger than the critical angle so the light doesn't escape from the fiber it reflects and so the light bounces around inside the fiber and comes out the other end and that's an optical fiber yes an optical fiber there's a couple of uses for that which I'll I'll talk more about these in other videos Fiber Optic Cables used for communication so they can carry digital signals in the forms of pulses of light so a digital signal is like in this case would be a light turning on and off and so these pulses of light can travel through these Optical fibers and carry information for examp example television programs yeah or telephone calls or whatever so communication and then the second one is if we have a bundle of fibers lots and lots of fibers together then that can be used uh in something called an endoscope and you can actually see inside Machinery you can see inside well it mentions on our specification uh see inside a person's body if you want to have a look inside their tummy to see what's going on without cutting them open you can put one of these endoscopes which is a bundle of fiber optic cables uh and you'll obviously need a light as well because it's quite dark in your tummy but we'll talk about that in another video as well so these are two uses of fiber optic cables and Fiber Optic Cables work by using total internal reflection