hi friends welcome to this class on refraction of light I am going to make the concepts really easy for you so I'm sure the topic of refraction of light is going to be crystal clear to you after you watch this class so friends be sure to watch the entire class hi good evening welcome everyone so great to have all of you here and guys if you haven't checked out our website do check it out manuchehr academy.com we've got these courses on physics for CBSE class ten CBSE class nine and ICS a class nine these are our full courses and we have great discounts going on right now so do check out the links below and we will also soon be launching the courses for chemistry and maths so guys stay tuned hi good evening everyone and if you haven't subscribed to our YouTube channel manoj academy hit the subscribe button right now and do it a life for this light class and share it out with your friends so that we can have more folks on these live classes so thanks a lot and thanks for your support I am happy to tell you that we are getting close to a 1 lakh 50,000 subscribers so we are going to be a family of 150,000 so that's kind to all your support and I'd really like to give a big thanks to all of you alright guys so hi I see lot of folks are out here so good evening hi ironhide Nazneen highest Vinayak hi Rinku convert hi dave history visa hi Akshay so very good evening to all of you and the topic for today's class is refraction of light so guys are you ready let's dive right into it so what is common to these pictures that you see here so here you can see can you see that in this picture the pencil appears bent when it's placed in water and you know that when you look at a pool a swimming pool it appears to be shallow right so it appears shallower than it really is and you must try out this interesting experiment so for example if you take an arrow as shown here can you see that arrow and if you hold a glass of water in front of it and then you look at the arrow what what will you observe you'll notice that it's Direction gets reversed so the arrow here gets reversed so friends you must try this interesting experiment it's like magic when you hold up a glass of water in front of an arrow the arrow actually this picture was pointing this way but when you look at it through the glass of water the avid erection as you can see is here so the arrow gets reversed and when we have when we wear spectacles right when we have power so where we wear these eyeglasses or spectacles right so on what concept do these work on so what is common to all the pictures over here what do you think guys so come on give a try hi good evening everyone so hi Tappan hi Saraswathi Gopal hi Gautham they've not hi on at the very good I see a lot of you have the right answer here so excellent so what is common to these pictures these are all based on refraction of light which is going to be the topic of this video so the pencil bends the glass of water due to refraction a boule appears shallower than it is due to refraction of light and that magic that you saw you saw there where they are will get reversed so do try it out that is also due to refraction of light and our spectacles or eyeglasses so these eyeglasses that I am wearing they work due to refraction of light right they help to correct our part and we can see more clearly great so now let's talk about this you might have heard this stuff before so before we go to refraction of light let's talk about this term the rectilinear propagation of light so what does this fancy six terms means it means that light travels in a straight line so this term means that light travels in a straight line now what is the condition so can you complete that sentence here for me so this sentence I'm talking about that light travels in a straight line when it is so what do you think is the answer yeah so guys what do you think is the answer this question or basically the question is asking under what condition does light travel in a straight line does it always travel in a straight line or maybe not right so under which condition does it travel in a straight line right so this is a rate some of you are saying so yes in this picture I have shown a ray right so as you can see this Ray's traveling in a straight line so when does it travel in a straight line very good I see in regular Chandra has the right answer here right or even now certain Kumar is saying that so when light is going through a single medium or when light is traveling through so we can say like this when it is traveling through one transparent medium so when it travels through one transparent medium then it travels in a straight line so let's understand what do we mean here so let's say the transparent medium is air okay so let's say it's traveling in air you know air is transparent light can travel through it and the word medium means material okay so this word means material and here our material is air so as long as light is traveling in one material material for example air or let's say it's traveling in one material like water or just through glass so as long as it's traveling in one material it's traveling in a straight line right so this is the important thing that rectilinear propagation of light means light travels in a straight line as long as it travels only through one transparent medium and we see what will happen when the medium changes okay so that's coming up and again before we go to refraction let's revise what is the meaning of reflection of light so earlier in the previous slide we talked about the rectilinear propagation of light right light traveling in a straight line now what will happen when this ray of light that you see here goes and hits a mirror so what we've shown in this picture here this is a mirror and you know mirror is a shiny opaque object rain so light cannot so opaque means the light cannot pass through the mirror but because it's shiny the light will as you know get reflected by the mirror right the mirror is going to reflect the light so all of you know that this mirror is going to reflect the light something like this right mirror is an opaque object remember that can reflect light right so this is the reflection of light which basically means the bouncing back of light okay and you can watch my video on reflection of light so you can search reflection of light Manoj Academy and watch more details about that and here what is the basic concept that if you draw normal right so there's going to be certain laws of reflection which we talked about so this is considered as the incident ray right so this is our incident ray here this is the reflected ray right and this is the angle of incidence here I and this is the angle of reflection R and you know that for reflection of light what is the condition I is equal to R right angle of incidence so the angle of incidence and angle of reflection are measured with the normal which is perpendicular 90 degree to the mirror surface so very good I see some of you answering that excellent angle of incidence is equal to the angle of reflection so this is the basic reflection of light and a mirror or a polished surface is a good example of that right now let's come to our interesting thing what is refraction of light okay so remember we said lightly you to travel so this light will continue to travel in a straight line as long as it's going in one medium but now something interesting is happening because this light was traveling through air right but now it hits this glass block here so what you see here is a picture of a glass block right and this light is of a single color so we have red light this ray of red light over here as you can see is going from there and it's going into glass it wants to go into glass and both of these materials you know air and glass are transparent they let light pass through and excellent I see lot of your answering here here the light will Bend very good so refraction of light means the bending of light so because light is going from one transparent medium from air in this case to another transparent medium it's not the same medium now it's going to glass so light is going to bend okay and the light will Bend something like this and we'll talk a little more about that how light bends but for now let's as you the light bends this way so if we draw the normal here you can see that there is clearly a bending of light okay let me should show it here in a different color so if we draw the normal like this right can you see that the light is bending and if you look here this is the angle of incidence and this is the angle of refraction over here so that is our angle of refraction and you can see those two angles are not equal so which angle is looking right area by this picture you can see so which angle you guys do you think is greater is it I or I what do you think so in this picture so very good it's bending towards the normal and excellent so the angle I is greater than and they are over here so you can see those two angles are not equal and light is bending and we talked a little more about that so now one interesting question is what is the cause of refraction of light okay why does light Bend as we saw her light was going from air into this other transparent medium glass so the real question is why does light Bend can you see the bending is happening where does it happen so the bending happens when light goes from one medium so please look here carefully the light was travelling in a straight line straight line straight line and the moment it hit here so when it's making a change of medium from air to glass you can see the light paint okay but the question is why does the light paint so good one part of the answer is it is due to the change in medium right so for bending of light light bends when light travels from one transparent medium to another but what is the cause of the bending okay so if they ask you in the test that what is the cause of refraction of light what will you write the answer okay excellent I see a lot of you have the right answer yeah so it is because of the change in speed of light right or change in velocity of light so it is due to the change in speed of light right when it goes from so when light goes from one medium so when it goes from one medium to another okay so this is a very important concept for you to know that it is due to the change in speed of light okay so what does this mean that lets say in air light was traveling at a speed off so let's understand with an example so in here you know the speed of light is 3 into approximately 3 into 10 to the power 8 meter per second okay and in glass the speed actually decreases so in air it's faster and then glass the speed is 2 into 10 to the power 8 meter per second so the light bends so this is the real cause of refraction now the speed of light can increase or decrease depending on the pair of medium so we'll come to that but please note the important thing if they ask you what is the cause of refraction it's due to the change in speed of light when it goes from one medium to another okay and so here you can see that it's going from air to glass and the speed changes so the light bends excellent so now let's come to the question we had asked how do we predict in which direction will the light men okay is it going to bend so if we draw the normal is it going to bend towards the novel or away from the normal how do we decide that right so let's understand that theory so let me first draw the normal here or first let's mark out so in this picture you can see the light is traveling from air to glass right and in this second picture here you can see the light is traveling from glass and it wants to go into air so these two diagrams are kind of opposite of each other right so can you see the light of the single color so we've taken light of a color red color here is traveling from air to glass or from glass to air right so now let's look at that first case here so this case one and this is our case two so let's focus on the first case where it's going from air to glass and now how do we predict that whether we know that life will not go in a straight line because there is a change of medium but will it bent towards the normal or away from the normal an excellent you guys are really interactive I am seeing the right answers here and so let's understand it let me explain you the concept why is it so because air is a rarer medium okay so if you compare air and glass air is the rarer medium and glass is the denser medium so just like in physics you heard about density which is mass per volume similarly there is this concept of optical density so density for light and they are not the same but usually the substance which is denser is also optically denser so light is denser right so for life air is a rarer medium right that means it is less dense so light can travel faster too where and when it hits glass glass you know is a denser medium ring and it's easy to remember glass is a solid ring and air is a gas so glass is a denser medium and when light hits glass the optical density is higher for it so light slow stop and the rule is that when light goes from a rarer to a denser medium so guys can you see here it's going from a rarer light is entering from a rarer to a denser medium so this way is going from a rarer to a denser medium and in this case the rule is it will bet towards the normal okay so take a careful look oops I will draw that again it disappeared so light is going to so light disappear there and so let's mark it here so what is the rule when it goes from a rarer to a denser medium it bends towards the normal okay so very important rule when going from rarer - denser it bends towards the normal and this case as you can guys can guess when it's going from denser because glass is a denser medium right and air is a rarer medium so when it is going from denser Carrera then this ray of light is going to bend a way from the normal so it's going to come out something like this okay so can you see so look here carefully at the picture here you can see the angle of incidence is larger and here you can see the angle of incidence is smaller and we say towards the normal because this ring so carefully look at the first picture the radiant go like straight like this it bent towards the normal yeah can you see so instead of going straight like this so like would have liked to go straight but can you see that it actually bent towards the normal and in this case it's opposite you can see that light would have gone straight away straight like this but it actually bent away from the normal because it was going from denser to a rarer medium so please important this remember this very important rule and you can easily predict based on that you just need to understand which substance is rarer or denser and so you can see clearly here that this is the rule okay so very good I see a lot of you have the right answers so the first one is towards the normal and the second one can you see going from denser to redder it bends away from the normal okay please remember this important rule very good now before we look at the laws of refraction let's quickly recap the laws of reflection right so you guys remember this that the incident ray the reflected ray and the normal at the point of incidence they all lie in the same plane right so remember that that when this is the incident ray Here I am drawing the reflected ray and now when you draw the normal right this angle of incidence is equal to the angle of reflection so we have I equal to R which is our second rule here so please take a look this was our incident ray the reflected ray and the normal they all lie in the same plane and the angle of incidence is equal to the angle of reflection now now let's look at the laws of refraction so this was for a reflection means this is for a mirror as shown here okay now let's take a look at the laws of refraction of light so you can see the first tool is pretty similar again we have the incident ray now instead of reflected ray is going to be the refracted ray and the normal right to the interface of the two transparent media so what what does that mean so here we are talking about the interface of the two transparent media so let me explain you the sentence interface means the line of separation so this line over here is called the interface okay and so when we say we are drawing the normal it's a normal to this interface here that is our ninety degree right so can you see it's normal to this yellow line right this interface if we just draw the line there so the incident rate the refracted ray and the normal to the interface at the point of incidence so this you know is the point of incidence right this point here can you see so that is a point of incidence they all lie in the same plane and let me draw the refracted ray now since it's going from a rarer to a denser medium it's going to bend towards the normal right because here again it's going from air and this is our glass block here okay so can you see that so now one important question is what is the plane here right so all of you can see the incident ray which I'm going to mark here so this is our incident ray this is the refracted ray can you see them right and the normal is that purple line which is perpendicular they all lie at the point of incidence they all lie on the same plane so guys let me ask you what is the plane over here right is it that interface or is it this surface that you're seeing here what is the plane that we are talking about so plane in physics you know is a flat surface thing so what is the plane here guys what do you think yeah I'm coming to the second law don't worry first we are right now discussing the first lawyer and then we'll talk about the second law so very good septum Kumar says it's the board right or it's the screen that you're seeing here so plane here means this thing is our plane so this entire surface that your seniors are not drawing over the diagram but this entire thing is the plane okay so this entire flat surface right here so this entire flat surface is the plane and you can see that the incident ray the refracted ray and the normal they are lying on this plane so when you draw this diagram in your notebook the plane is your notebook okay so a lot of students have confusion regarding this what is the plane so same thing just like in law of reflection this is our plane here the flat surface so the screen or if you are drawing the diagram in a paper it's the paper same way over here okay so please remember that excellent and now let's take a look at the second law which is Snell's law which states that the sine of the angle of incidence so remember the angle of incidence just like reflection it's the ins the angle the incident remakes with the normal so for refraction we define it like that and now instead of reflectively it is the refracted ray right so it is there this is the angle of refraction and we are saying that the sine so this trigonometric ratio right the sine I by sine odd so when you take this ratio it's a constant so let's really understand and this law which is known as Snell's law so these are the two laws of refraction that you need to learn up and I'm going to discuss and make the second law also clear to you so let's talk about this law which says that so I need to quickly draw this diagram here again so this is our norm you guys right and this is our refracted ray here it's traveling from air to glass so you can see it bends towards the normal and this is our angle of incidence and this is our angle of refraction right so is that clear the angle the incident ray and the refracted ray make with the normal now this ratio we say is a constant so let's understand what this means so let's take a look at the table here let's see the first time we do this experiment so imagine you're doing this experiment and you have a laser beam or something you're throwing red light right on this glass slab and the first time the angle of incidence is 30 degree right and then you measure so you can measure that angle with the protractor and then you do the experiment and let's say you check the refracted ray that comes out the angle of refraction as you can see in this table so the angle of incidence is given as 30 degree and the army of refraction is given as 19 degree so now we need to take according to Snell's law it's not the I PI R ratio it is Sinai by sine R and remember max what is the definition of sine sine you know is it is perpendicular by hypotenuse it's a trigonometrical ratios right so it's this trigonometric ratio and we need to calculate it for these angles so let's go ahead and do the first one so I'm going to take the help of the calculator here so let's pull it up right so we are going to do Sinai by sine R so let's take so in our first case the angle is 30 so let's take 30 here and be able to do sine of that so sine of 30 which you know is 0.5 1/2 right divided by sine of 19 no that's not a standard value so we are using the calculator here so divided by sine of 90 so guys you can see I'm doing this division and let's see what do we get one point five three so I'm going to approximately write the ratio as one point five okay so guys you saw her we are doing this Sinai by sine R and we've got this value out here one point five right now let's do it for them let's say you do the experiment again so now what does this mean so now we increase the angle of incidence so what this means is the next way is something like this okay so you're basically turning your laser pointer or you are changing your source of light so that now the angle of incidence increases to 45 so this is our new eye and you can see we are doing it with 45 okay and then of course the new refracted ray is going to come out at a slightly larger angle here so I'm just drawing the rough diagram and we saw that the angle is 28 degrees so let's say you did this experiment and now it came out to be this so again let's go back to our calculator and do so according to Snell's law we need to do sine of 45 so very simple just take sine of 45 which you know is 1 by root 2 or we abuse the calculator value divided by sine of 28 okay and what are we going to get so there you see what is the value that you got here so take a look at this so what do you get here one point five zero again right so it's one point five okay excellent and let's try the next one which some of you already guessing right so what it's going to be because by Snell's law it should be same so it's going to be 60 degree so you can imagine that it increases again so sine of 60 your noise root 3 by 2 so we use the calculator value divided by sine of 35 so guys I'm showing you the real division here and there you can see what is the value guys we've got excellent can you see 1.5 and of course it's approximate because these angles are not measured accurately but the lowest perfect that if you dip the ratio you would get exactly the same thing but you know it experimental there will be some experimental errors you won't be able to exactly measure the angle so there can you see that we've got Sinai by sine R it is constant and we have got the value as 1.5 right so what Snell's law says that for a pair of medium okay so this is for a given pair of media or mediums right so for a given pair of media and in this case what is it air and glass this is our example here so it could be here in water water and glass whatever for a given pair of media so it's not that the value is going to be constant always it's not just one standard value for air and glass so for this pair the value is 1.5 okay so can you see that so for this pair the value turns out to be 1.5 and you've seen that we've done the table right you try it out all these values and we did the division I showed you in a calculator so I would encourage you to use your mats or for these difficult angles you can use the calculator and try it out you have sign in the calculator and when we divided the ratio we got the same value 1.5 so Snell's law is absolutely right he says that when you do the experiment for a pair of media and you measure the angle of incidence and refraction then the ratio of the sine of the angle of incidence by the sine of the angle of refraction as you can see here should be constant not I by our sign I by sine R okay so please remember now I hope this noise clear to you right because a lot of people have confusion about what this law is talking about so it's basically changing the ray of light so for the third so this was for our first value in the table this is our second and the third rate will be larger angle right so I roughly drawn it here and so you can imagine it'll come out something like this so you keep changing the source of light measuring the different angles INR finding the ratio and then you get it right excellent so I hope you got this and guys do you know what is this ratio known as so what is this ratio quantity so what do you guys think do you guys know so this ratio of sine I by sine art what is it known as so someone's asking how to get sine 28 of course they are match tricks for that but here we were using the simple calculator right so you use sine because we are checking the law right so you just use sine of 28 and as I showed you you can pull up the calculator it's the trigonometrical map use your name so you just write 28 over here right so let's clear the calculator 28 and somebody is asking how to take the sine of that you just press on the sine button here right and that's how we did the division okay so now can you tell me guys what is this ratio known as very good I see someone has the right answer here right so there's this ratio is known as refractive index okay and it is the this refractive index is constant as we discussed for a pair of media and let's talk about this term refractive index so the Snell's law is basically saying that the refractive index of a pair of media sinai by sine R as we discussed is equal to constant okay so what does this mean so if you take any two media right so let's say this is medium one and this one is medium two right in our case it was air and glass but it could be any medium strain so when light is traveling from medium one to medium two so note this carefully then we devote the refractive index like this in to one can you see that notation so what does this mean let's understand it this is the refractive index of medium 2 with respect to medium 1 okay please understand this notation n to 1 means refractive index medium 2 with respect to medium 1 there are other different notations some books use the Miu symbol right and you can even write it as mu to 1 like this or n to 1 like this straight or like we've written here it could be in 21 it looks like a 21 or a Mew 21 train so these are all meaning the same thing but note the sentence what is here so when you write 21 or two on top and one below it is the refractive index of medium 2 with respect to medium one so refractive index of the second medium with respect to the first medium when light is going from 1 to 2 okay so please note this definition carefully because a lot of people have confusion in this is it respect to 1 or is it to with respect to 1 or 1 with respect to 2 so when we write n - 1 or a name 21 or any of these notations that I've shown you here these all mean refractive index of medium 2 with respect to medium 1 okay so - so you're comparing glass with respect to air or medium 2 with respect to medium 1 here okay and like here we saw the light will go and repent towards the normal right but that doesn't matter which medium we're talking about it's always second with respect to 1 when light is going from 1 to 2 and so he Snell said the Sinai by sine R is constant and there's also another relation with the velocities because remember we talked about refraction is due to change in speed of light or change in velocity of light so let's say the velocity of light in medium one is V 1 and the velocity of light in medium 2 is V 2 then we can say that n - 1 so I'd like to write it this way right refractive index of 2 with respect to 1 that can be written as the ratio of the velocity in medium 1 not in medium 2 so it's very confusing divided by the reeva lhasa t medium - so guys please understand this part carefully that when you are looking at in terms of velocity we like V 1 by V 2 and that is also equal to so this thing is equal to sine I by sine R right so an easy way to remember it is so whether you take the velocity the ratio of the velocities or the ratio of the sine of the angles it's always medium one with respect to medium two so remember this was our angle I was here and R is here so guys look here follow me carefully if you look at the refractive index of two with respect to one it is velocity medium one right so the velocity in the first medium divided by the velocity in the second medium so that is one way to define refractive index this ratio and the other way to define it is into one according to Snell's law is Sinai by sine R so sine of the angle in the first medium so angle of incidence divided by the sine of the angle of refraction right and usually the definition of refractive index is actually taken in terms of the ratio of velocities okay because the more fundamental thing is it's a ratio of the velocities the speed of light and medium one by the speed of light in medium - and that happens to be related so there's a proof of that it happens to be related to the sine of the angle in medium one by the sine of the angle in medium - okay clear so this is how you do this thing and if you take this example here so like we said the speed of light so let's say the medium one is air so this is going to be 3 into 10 to the power 8 meter per second right and this velocity let's say glasses 2 into 10 to the power 8 meter per second right so let's use this refractive index formula and let's find it out so in glass with respect to air because 2 is glass here and one is here is going to be 3 into 10 to the power 8 meter per second right sorry that's going to be 10 to the power 8 meter per second divided by 2 into 10 to the power 8 meter per second right and so if you work it out and meter per second will get and these 10 to the power 8 gets cancelled so what is the value again again 3 by 2 which is 1.5 okay so guys please follow me here can you see that that we are getting the refractive index of glass with respect to air so medium 2 with respect to 1 is working out to be 1.5 okay then we took the ratio of their velocities so 3 into 10 to the power 8 by 2 into 10 to the power 8 and then we used our sign table also you saw we got the same value 1.5 so this shows that refractive index can be defined as ratio of velocities or the ratio of the sign of these angles okay so I hope this concept is clear otherwise go back and look at these formulas I'm sure it's going to become very easy for you and just follow the way I've explained it right the refractive index of medium 2 with respect to 1 is defined this way okay so this is a very important concept for all of you to know now let's talk about refraction to a glass slab okay so we're what we interested in is that light is traveling from air to glass and as you expect that the light is again gonna come out into air right so this is our incident ray here okay so this is our incident ray and it is the red light so light of a single color we're not using light of the white light or multicolored because there's another term you may be knowing that this version of light so I could split it into its colors right into the different rainbow colors so that's why for this discussion we are keeping light of monochromatic means single color so we are just using red light away on now let's take a look what will happen when it goes to a glass slab okay so first thing is pretty easy we know that when light travels from air to glass the light is going to bend towards the normal because it's going from rarer so air is rarer to a denser medium right and now the interesting thing is what's going to happen when light goes from glass to air okay so what will happen here is the interesting thing over here so you can see them so what do you guys think will happen will light bend towards the normal or away from the normal at this point here so at this point what do you guys think will happen so when light is coming out of the glass slab will it bent towards the normal or away from the normal so note here light is going from glass to air so guys what do you think is going to be the answer yeah very good I see some of you are saying the correct answer light is going to bend absolutely right away from the normal because we are using that simple rule that light is now travelling from a denser because glass is denser than air so since now light is going from denser to a rarer medium it's going to bend away from the normal okay and what are the terms use here this is called the refracted ray the Ray in the glass lamp and this ray that comes out is known as the emergent ray because it's the ray that comes out so there you can see the important terms incident ray refracted ray and emergent ray Tim very good and now what is this this angle is called the angle of incidence as you know so this angle is the angle of incidence now this angle here will be the angle of refraction okay and the angle that the light comes out with I'm going to mark it here in yellow this is the angle of emergence okay so now guys can you tell me what is the relation between the incident ray and the emergent reign of glass slab so there's a very interesting point here which we are going to discuss what is the relation between this incident ray and the emergent ray or we can say what is the relation between the angle of incidence and the angle of emergence so guys what do you think so I want all of you to try and look here what we are doing we are doing the topic very important topic refraction through a glass slab so when light is incident on a glass slab first it's going to bend towards the normal because it's going from - glass Raider - dancer and then when it's going to come out now there's a change in medium from glass to Elle it's going to bend away from the normal okay so guys what do you think is the relation between the angle of incidence and angle of emergence are they equal or is I greater than E or is greater than I so what do you guys think very good I see a lot of you are getting the right answer so many of you are saying I equal to E okay so let's understand that can be proved that here that is the angle of emergence equal to the angle of incidence so let's take a look so to prove that please follow me carefully can you see that this is the angle of incidence I here and this is the refracted and they are okay so now our mystery angle is what is this sound they're gonna be the angle I'm gonna mock here right so what is this angle gonna turn out to be right the angle this refract agree is making with the normal when it comes out so let's understand that so we can see that these two normals are perpendicular to the glass slab right and we know that these lines are parallel right because the it's a parallel glass slab so these lines here are parallel the edges of the glass slab and so these normals which are making 90 degree to the glass slabs they must be parallel because they make the same angle 90 degree with parallel lines so we can say both normals here so both the normals in the diagram are parallel right so both normals are parallel now what can you see if the two normals are parallel we can apply the interior alternate angles right so this angle R that the refracted ray was making in the first case in the first refraction is going to be the same angle here so I'm going to remove this question mark and I am going to write R here this is by interior alternate angles because these two normals are parallel lines right so these are parallel lines so they are making the same angle can you see this interior alternate angles so this one and this one they're going to be R right so both these angles saying now we can say that when light is going from air to glass right if it has a pair of angles ie and R so the angle of incidence was I and the refracted angle is are so normal you look at the second case when light is going from blast to air it will again at the same pair right because the angle of the glass is the same are here so the angle in air is going to be exactly same I so that is the trachea right since these are same pair of angles so follow me again when light is going from a two glass in the first case the angle of incidence is ie in air and are in glass right now when you look at the second case again now light is going to the reverse way from glass to air but the angle in glass is same R so the angle in air is also going to be same eye and so therefore we can say the angle of incidence is equal to the angle of emergence excellent so you guys got that right very good and so angle of incidence is equal to the angle of emergence and now that brings an interesting point since we said that these edges of the glass slab are parallel right now the incident free and emergent rain make the same angle here which is I am so they are making the same angle who have already to do that hi here so they are making the same angle with these parallel lines so therefore if two lines make the same angle with parallel lines therefore we can say that the incident right incident and emergent rays are parallel this is a very important concept that the incident ray and emergent rays so in incident ray and emergent rays are parallel here so let me write that okay so that's very important so what is happening here the the glass slab is not really bending the light it is shifting the light why because if you see this was the original path of light so if there was no class tap the glass light would have gone like this but you can see that there is a shift of light and this is known as so this thing term is known as laterally displacement okay so there has been a lateral displacement of the light so instead of the light going straight here it has have it's having a displacement right so the light has shifted over here and this is the important point that incident and emergent rays are parallel and so there is a shift in the light okay guys so this is a very important topic usually questions come from this part refraction of a glass slab so you must really understand this diagram and I've made it really easy for you by showing you the proof of why angle of incidence is equal to angle of emergence it's a very simple geometric proof with parallel lines and alternate angles so go ahead and practice it so that you get familiar and find it easy and then therefore we can say the important result is angle of emergence and angle of incidence are same and therefore incidentally and emergent areas are parallel so there is only a lateral displacement of the light for a glass slab okay so excellent now let's take a look at this special case so guys can you see here there's this ray of light is that normal incidence so when it's traveling from air to glass it's being incident at a 90 degree angle here so can you see so what do you think will happen here so this is a question for you guys to try so guys what do you think will happen will the light Bend with the light get reflected will the light go straight or is it none of these guys what do you think here so what do you think guys is the right answer for this case please try it okay so I see some of you are saying light reflects light go Street very good so please try so let me talk about one important point here which a lot of times is not mentioned in the books so let me talk about that before we discuss this question so when we looked at this our focus was on refraction of light right the bending of light but you must be knowing from your experience that when light hits a glass slab there is also a partial reflect reflection okay so this we didn't talk about because our focus was on the refracted ray we were focused on the refracted ray refraction but there is also partial reflection here or a weak reflection because we know glass is not like a mirror it's not a good reflector of light so there is a partial or sorry let me write a reflection here right so there's a partial reflection so in some textbooks you might see this complete diagram where they talk so we're focused on the reflection of light but you know that when light will hit from air to glass there is going to be a refraction and a partial reflection so remember this important point and here angle of incidence really equal to angle of reflection so they didn't follow the laws of reflection here okay so this is the complete diagram for light going from eight glass there's going to be a partial reflection as shown here and a refraction okay but most of the time when we focus on refraction we ignore this guy gets ignored usually we don't talk about it and we are focused on only the refraction so now let's come back to our question here right so the question was asking what will happen when light is at a normal incidence which means can you see normally means 90 degree so we'll light bed light reflect will it go straight or none of these so what is the best option here and my hint is we are discussing the topic of refraction right okay so our focus should be on the refraction of light so now these two that can you tell me what is the best answer here so super I see you guys Rock you guys are absolutely right the C is the correct answer light goes straight so this is a very special case where light will simply go out straight so it will come out again it will be 90 here right and 90 here also when it comes out so light goes straight here so let's look at the options there is actually going to be a partial reflection here but that's not our important case so that's why we are not selecting this option even though a partial reflection is right we'll be looking at the best option here light bends so the interesting thing is in this case light does not Bend now why is that because of this special is where you can see if you draw the normal the normal is going to be along the light train so in this special case angle of incidence is going to be 0 degrees the angle the light makes with the normal an angle of refraction also is going to be 0 so very good this is a special case where angle of incidence is equal to the angle of refraction and light goes in a straight line it does not paint okay when it comes out even back in air so in this special case guys please remember when it's a normal incidence angle of incidence is equal to angle of refraction and same thing happens here I is going to be 0 R is going to be 0 for this special angle it's going to turn out to be so I'll write that once more here if you can see so I 0 and R is 0 here right so that is our special case where light comes out in a straight line but we talked about usually when you hit light at an angle so when it is incident at an angle like this right so an angle like this or we saw these angles like this light bends so we say that the light is incident at an oblique angle so an angle which is greater than 0 then you will see the bending but in this special case there is no bending light goes straight so very good guys you got the correct answer please remember this special case now one interesting thing is does the speed of light change here so if I ask you guys does the speed of light change for this case so let's look at the first example when it's going from air to glass so in this is the speed change when it's going from air to glass so what do you think so C is the correct answer for light going straight but does the speed of light change or not what do you guys think so somebody's saying no what do you guys think here so I'm asking the question when light goes from air to glass will it speed change yes or no okay so very good I see Milan and muku Kumar they have the right answer a lot of you Gotham they've not shares very good so because the rule is light is going from a rarer right to a denser medium so the speed of light is going to change but just because of this special angle of zero it does not bend so please remember this important concept that because light is traveling from air to glass the speed will change and again when it travels from glass to air the speed will change right if you compare air and air it will remain same but in this when the transition is happening from one medium to another speed is gonna change because light is going in hitting a different medium okay so does the speed change the correct answer is yes but the light does not paint okay so remember this interesting question where speed changes but light travels in a straight line excellent and now I've got this homework question for you so as you can see in this question when light travels from glass so can you see so this is the gas to water so the question is when light is traveling from glass to water as shown in this diagram does it a bend towards the normal does it Bend away from the normal or C does it it does not Bend or D it depends on the color of light here we are using light of the single color red light and to let me know your answer by putting it in the comments below so I look forward to reading your comments and I will reply to them as soon as possible so guys think about the concepts that we've learned in this video applied to this question and I'm sure you get the answer so you need to decide will light bend towards the normal away from the normal does not Bank or it depends on the color of line and please write your answers in the comments below I look forward to reading your answers and I will reply to them as soon as possible excellent I see some of you are writing the answer here guys you've been really interactive and participating in this video I've thoroughly enjoyed it but I want you to write the answer in the comments and I am NOT going to give the answer I want you to try it out and you can take out more refraction videos on our website so we have the concept of refraction of light in more detail and the refractive index topic is dealt with more detail with the different formulas here we just touched on the basic concept that there are lot of important formula involved so do check out our videos in the course on our website which is Manoj Academy calm so these are under the physics class ten course you will find them and I'll put the links below and guys as I said we have big discounts going on on these courses so to avail that discounts I'm sure you find these courses very useful and guys hit the like button and share out this video with your friends and to write your comments and feedback so we are taking these live classes on physics chemistry and maths and hit the notification bell so that you don't miss out on our classes on this Vannucci Academy Channel and share it out with your friends so that we can have more people here and to check out this website muchacha de me calm thanks a lot guys for your support and it's big thanks from our team because we really enjoy reading all the comments and we want to bring more awesome content for all of you so hope you enjoyed this class and thanks for being really interactive and participating in this class guys I really enjoyed it okay all of you take care thanks a lot and yeah do try that homework question and write it in the answer in the comments below your answer okay guys bye take care