 Can you guys hear me? Okay, great. I'll wait for few other people to join and then I'll start the class. Basically in this particular class, share all the doubts that you have in physics till now. I'll try to cover it up as much as possible. The class particularly will be focused on revision for your upcoming test. And I'll not be taking anything which is not related to your upcoming examination. So no mechanics and all those things which has been written in the schedule. So I'll start doing the revision once everyone joins. So only 4-5 people are here till now. So can you guys tell me which topic I should start with? Okay, Roshan is saying optics. So few of the things I have already done in optics. But I have written a question. I'll not only do numerical best questions. The reason behind it is that in your exam only numeric will not be asked. In your exams theory will also be asked. So I will be taking some theory also. So if you want to do numerical write, I'll give you a numerical. Okay, people are not joining, I don't bother. It has been too much of time. Write down a question. It's based on where I'll come to magnetism. I'll definitely come to magnetism. Don't worry about it. So let me first solve a few questions from optics and then I'll come to magnetism. Because these are the two topics you guys have been asking for. Write down the question. The question is a coin is placed in a big up depth of 3 centimeter from water surface. So it is not placed. Try to understand. It appears in a bigger at a depth of 3 centimeters. Find the real left. Mu water is equal to 4 by 3. As soon as you get the answer, you can post it in the chat. What's the answer guys? Nobody is doing the answer. Okay, one more minute, I'll wait. Seven is a wrong answer. Do it once again. No, that's wrong. Yes, it's correct answer 4 centimeter. It is correct. Your answer was correct. Okay, guys try to understand. I'm not getting into derivation now. The formula is a refractive index of medium is equal to the real depth divided by apparent. So the refractive medium of water has been given as 4 by 3. That is equal to the real depth divided by apparent depth has been given as 3. This and this one real depth is equal to 4 centimeter. Now the question should be why there I mean from where does the concept of apparent depth come comes from. So that I will explain you what happens is suppose I am placing something over here and this is filled with water. So this is water on this side. So whenever a light ray travels from suppose this is the normal. So suppose my eyes are here. So what will happen? The light will travel from here. The light will travel from here and attitude it is going from denser medium to rarer medium. It will diverge or it will go away from the normal. So what happens whichever ray reaches my eye. The object would be located for my eye object would be located in a straight line to the ray which is reaching my eye. So as my eye is here and refracted ray is reaching my eye. Hence the object will seem to be or appear to be placed over here. So that is why this is the real depth. And this is the apparent depth and the formula or the equation between or formula is mu of medium is equal to real depth divided by apparent depth. This is how it is. Okay I will give you one more question. Now here only one medium is there. Suppose I keep two mediums then what happens? So I draw the question. The question is I will just draw the diagram of the question. So instead of one medium I have two mediums. This is three centimeters and this is two centimeters. And mu of this medium suppose this medium is B so mu B is equal to four by three. And suppose this medium is A mu A is equal to three by two. And the object is placed over here at the bottom here this is the object. I am asking what will be what will be the vertical shift in the object in the image of the object. I will explain how did I get it. After this question I will explain. Solve this question. What is the answer guys? Okay great problem solve it. Okay what is the answer? No 8 by 3 is not the answer. Okay tell me one thing guys should I tell you the derivation of how do we get the formula of mu medium is equal to real depth divided by apparent depth. You want me to do so? Yeah great. So the answer is three and a half the apparent depth. I am asking the vertical shift. So vertical shift would be five minus three and a half because see what I do is that apparent depth I will be calculating. Apparent depth I will be calculating from here. So apparent depth is calculated from top. So from answer from top the answer would be three and a half centimeter. Vertical shift is calculated from the bottom. So vertical shift would be if this length is three and a half vertical shift would be five minus three and a half. But how do you do so? So try to understand guys the formula over here is if more than one medium is there apparent depth is equal to d1 divided by mu1 plus d2 divided by mu2 and likewise. So here d1 is three divided by three by two and d2 is two divided by four by three. So how much it comes? It comes out to be two plus three by two which is one and a half. So apparent depth comes out to be three and a half centimeter. As soon as you get apparent depth vertical shift would be equal to five minus three and a half which is equal to one and a half centimeters. Is it okay? Anyone who wants me to derive the formula I can derive it for you. So let me know otherwise I'll move ahead. Okay great Param so look at here how it is done. There are different methods to derive it but try to understand. So I'm doing it here. Suppose I have a line like this and I have an object kept over here. So let's draw a normal and from the normal a ray will go like this. This is I. If this is I this will also be I. Now a ray goes like this and this is R and here R is more than I. And due to this R what will happen is the object will appear here. So if that is R this would be R. So this is my apparent depth HA and this is my real depth HA. So what happens over here is let's write Snell's law. So by Snell's law suppose this is mu medium. So Snell's law is mu medium and sine value of angle in that medium. So that is sine I and mu air which is one into sine R. So what I get is what I get is mu medium is equal to sine R divided by sine I. And now what I need to do is I need to find out value of sine R and sine I in terms of HR and HA. Now guys try to understand when I value and R value are very less. So in that particular condition what happens is suppose look at here. Suppose this is the point and I have this particular angle. So if the angle keeps on reducing like this, this length and this length keep on becoming. So hypotenuse if you keep on reducing the angle the length of the hypotenuse will start becoming the length of the base. So in that particular condition if I is very less and R is very less. So in that condition or what you can do is that directly you can find out value of tan I and tan R even without doing that. So because here sine R and sine I is there I will be writing here sine I will be equal to tan I and sine R will be equal to tan R. So what happens over here is I write tan R here divided by tan I. Now if this length is X tan R would be X divided by HA and tan I would be X divided by HR. So if you solve it it becomes X by HA into HR by X. X X gone this comes out to be height of or depth of real depth divided by apparent depth and that is equal to mu of medium. Is it okay? Yes or no? Are you understanding? Yes or no? Okay no issues. Now what is the vertical shift guys vertical shift is this this is the vertical shift. So vertical shift is try to understand here vertical shift is equal to HR minus HA. So try to understand I will represent vertical shift in terms of HR. So to do so I need to reduce HA or I need to remove HA. So what will I do? I take a reciprocal of it so it becomes HA by HR and that becomes equal to 1 by mu M. I subtract both from 1 so what I get HR minus HA divided by HA is equal to 1 minus 1 by mu M. Now this HR minus HA is vertical shift so vertical shift is equal to this is HR guys I mistake I have written this. So HR is vertical is equal to HR 1 by minus 1 by mu M. Is it okay? So if you if suppose I have to find out vertical shift in this case. So what I can do is that I can calculate vertical shift directly. So vertical shift will be added by all the mediums. So here the vertical shift would be 3 centimeter into 1 divided by mu of medium. So that becomes 2 by 3 so how much it comes 3 into 1 by 3 this comes out to be 1 plus. Here this is 2 1 minus 4 by 3 which becomes 3 by 4 so this comes out to be 2 into 1 by 4. So this is 1 by 2 so this comes out to be 0.5 so vertical shift comes out to be 1 and a half centimeters. So that's how you can solve this question. Did you all understand it? Okay great. Okay solve another question. Another question I will give you. Wait for a moment. Okay write down a question. I am giving it on the lens. So yes Param you can directly add because what happens image of this particular image formed by this particular medium would act as an object for this particular medium. So if you derive it it will come out to be total vertical shift is vertical shift due to both these mediums added. So resultant vertical shift is addition of vertical shifts by all the mediums. It can be derived here. It's same as the other derivation where i is very small so i would be equal to r and you can find it out. Next this question is convex lens of f is equal to 0.12 meters reduces an image which is 3 times the size of our object. You have to find out value of u. The hint in this question would be that I have not given you whether I am finding out real image or virtual image. You will have to consider both the cases. One for real image and one for virtual image. No Param answer is wrong. Which answer? Rushal your answer is correct. Param your answer is wrong. But there would be two answers as I told. The first answer would be for real image. The second answer would be for virtual image. What's the answer guys? I got only one correct answer. Have you guys got it or should I solve? Let me know Param what are you doing? One meter is 100 centimeters not 1000 centimeters. Okay one and a half more minutes then I will start solving. So here your answer is for virtual image or real image. So if you are giving me answer write down in bracket whether the answer is for real image or virtual image. Yes Rushal but I am asking real or virtual. Okay great. Okay fine guys let me solve now I got two three answers now the rest of the people are not responding. For real image in case of convex lens we would be positive. So in that particular case first let me find out the magnification. So for magnification I am solving here. Magnification is equal to m h height of image divided by height of object is equal to v divided by u. So v by u comes out to be three because it has been given images of three images three times. So v by u is equal to three. Now using this what I do is that so try to understand. Let me use the lenses formula one by f is equal to one by v minus one by u and let me find out the relationship v is equal to three u. Now here what I do is that focal length will always be positive. So I will make it this is point one two meters while make it centimeter this is 12 centimeters one by v would be positive. So I'll take one by three you and this is you will always be negative. So this becomes plus one you. So this comes out to be one by 12 is equal to four by three you is equal to four into 12 divided by three. And this comes out to be 16 centimeters for a real image for a virtual image. The relationship remains same but the v would be negative. So I write minus one by three you and this will always be positive. So this comes out to be one by f is one by 12 and one minus one by three is two by three you. So you will be equal to 12 into two divided by three. So that will be equal to eight centimeters 24 divided by three. So that is how that is what the answer should be. Now let me give you a question. The question is like this. So write down the question and orating the question just write it down. It's taking a lot of time for me to write down the question here. So I will orate the question you just write it down. Listen to my question carefully to convex lenses a and b. It's a convex lens. So there are two convex lenses to convex lenses a and b of focal lens 30 centimeter and 20 centimeter respectively. What are the focal lens focal lens are 30 centimeter and 20 centimeter for a convex lens a focal length is 30 centimeter. For convex lens b focal length is 20 centimeter. These two lenses are placed in a contact. The question is calculate the effective focal length and power of the combination. The first question is find out the effective focal length and power of the combination of two convex lenses. This is the first part. I'm again repeating the first part. The first part is calculate the effective focal length and power of the combination of two convex lens. The second part of the question is if I if b is a concave lens. The second part of the question is if b is a concave lens rather than a convex lens. The focal length remains same that is 20 centimeter. I'm just changing the nature of the lens. The nature of the lens was convex in first scenario. Now by changing it I'm making it a concave lens so b becomes a concave lens. So again you have to find the two things effective focal length of the combination and power of the combination. I'll again read the question for you two convex lenses a and b of focal lens 30 centimeter and 20 centimeter respectively are placed in a contact. Calculate the effective focal length and power of the combination. Second part is if b becomes a concave lens then what is the effective focal length and the power. Yes the unit was centimeter 30 centimeter and 20 centimeter. What's the answer guys? So I've got two three answers. Russia's answer is correct but for only first part. Shia's answer is correct for Shia your answer is not correct for the second part. The second part the first part is correct. Okay you are giving focal length and okay fine. Both your answers are correct. Russia you check your answer. Second part you check your answer. All of you check your answer. Shia your answer is correct. Russia you check your answer. Answer is correct Russia in the first part. One more minute others. But the first part answer is 8.33 and 12 centimeter. There is no doubt about it. I'm asking the answer for the second part. Okay. Focal length for a combination of lenses is 1 by f1 plus 1 by f2 plus 1 by f3. So first one is 30 centimeter and second one is 20 centimeter. If both are convex then the focal lens would be equal. Sorry positive sorry positive. So focal lens would not be equal there 30 and 20 there would be positive. So this is 60 this becomes 2 plus 3 5 divided by 60 that is 1 by 12. So focal length comes out to be 12 centimeter. Now power is defined by 1 by f and f should be in meters. So what I do I do 1 divided by 12 centimeters which is 0.12 meter which is 100 divided by 12. 100 divided by 12 would be 25 divided by 3 which is nothing but 8.33. Now what happens is in second part. So let me solve second part here somewhere. So again I will write 1 by f1 is 30 f2 becomes or lens B becomes concave. So the focal length will become negative. So instead of this 20 positive I'll take 20 negative because nature of the lens has changed. So this becomes 62 minus 3 so minus 1 by 60. So f comes out to be minus 60 centimeters. So power would be 100 divided by minus 60 that would be equal to minus 5 by 3 which is nothing but minus 1.67 d. If you don't write this minus the examiner will cut all your marks. Why this question has been given this question has been given to test your knowledge of positive power and negative power of a lens. So if combination is concave the resultant power would be negative. If combination is convex the resultant of the combination is convex the power would be positive. So that's how it is. This is how it will take a lot of time that's not required for now. So we'll waste a lot of time sometimes in the class after your exam I'll derive that formula don't worry about it. Now just remember it. It will kill a lot of time. I'll do this formula in the class 1 by f is equal to 1 by f1 plus 1 by f2. Next question I'll take questions based on mirrors also. So this is this is A this is B this height is negligible. I'm not giving any height. This height is negligible just for the sake of proper interpretation I have made like this. This height is negligible in nature. So length of this rod is 10 centimeter. And it is lying along the principal axis of concave mirror of focal lens f of this mirror is also 10 centimeters. And this is 20 centimeter. You have to find out image of the rod. One or two more questions and then we'll move to another topic magnetism and then we'll start solving questions from there or we'll take theory first and then we'll start solving questions. Do you guys understand moving coil galvanometer and all those things properly? I'm asking do you guys understand moving coil galvanometer conversion of moving coil galvanometer into ammeter voltmeter all those things properly. The focal length is 10 and this is also 10. Paran you are right. Others what's the answer guys? I'm not getting any answer. Paran it's not 15 centimeter. Just check once all of you. You guys are giving me value of V. Yes it's 5 centimeter. Now I'm getting the answer. Look at here guys what we need to do is that we need to find out where image of this end would be formed and where image of this end would be formed. So mirror formula is 1 by V plus 1 by U is equal to 1 by F. Now 1 by V this is 20 centimeters. I'm taking negative 1 by 20 and this is how much minus 1 by 10. So 1 by V is equal to minus 1 by 10 plus 1 by 20 which is equal to minus 1 by 20. So V is equal to minus 20. It means that image of A would be at the exact location of A. So A dash would be here only. So image of A would be at its position only. Now let me go to image of B. So V minus this is 20 plus 10 30. So this is this becomes minus 30 and 1 by F is how much minus 1 by 10. So 1 by V is equal to 1 by 10 negative plus 1 by 30 and that is equals to this is 30. So how many times it goes? It goes minus 3 times plus 1 time which is equal to minus 2 by 30. So which is nothing but minus 1 by 15. So VB would be equal to minus 15. It means that B dash would be here somewhere from here 15. So this is at 20 centimeter and this is at 15 centimeter. So distance between them which is the length of the rod is, length of the image of the rod is 20 minus 15 which is 5 centimeters. So that's how it will come. Okay, next question. An object of height 2 centimeters is placed at a distance 2.5 F from a concave mirror. So you have to find out height HI when this F is focal length. Answer quickly. Answer, yes. Russell you give me correct answer but you need to work on the sign rules. I mean whether it will be positive or negative that you need to mention in your answer. So sign conventions has to be taken care of. Yeah, so you should write minus 4 by 3. The examiner will automatically understand that it's an inverted image. Okay, other answers. One more minute. Before I start magnetism, I would like to ask you what topic in magnetism you guys would like me to cover. Okay, DC motor, AC generator. Okay, brightness of the bulb. I don't know whether I'll be able to cover it till 515 because I have another class. So whatever I would be able to cover up, I'll try to cover it up. Okay, let me solve this question for you. So here 1 by V plus 1 by U is equal to 1 by F. 1 by V minus 1 by 2.5 F is equal to minus 1 by F. I'm writing it 1 by V minus 2 by 5 F and that is equal to minus 1 by F. So 1 by V is equal to 2 by 5 F minus 1 by F and this comes out to be minus 3 by 5 F or V would be equal to minus 5 F by 3. So this is what the answer is. Now what about magnification? Magnification is HI divided by HO and here it is slightly different from the lenses formula. Here it comes with the negative sign. So V divided by U. HI would be equal to minus HO V by U and HO is 2 centimeters. V is minus 5 F by 3 divided by U is minus 5 F by 2. So what happens over here is this and this gone, this 5 and 5 is gone, this F and F is gone. This 2 will go in the numerator. So this will come out to be 4 by 3 only. So this will come out to be 4 by 3 and I have left a negative I think. Yes, this negative I have left here. So this will be minus 4 by 3 centimeters. So that's the answer. Now let me move to magnetism. And before that let me check what I need to cover it up over here. So as I move into magnetism, first rule that I discussed is inverse square law of magnetism. What is inverse square law of magnetism that I will first explain. So suppose I have a magnetic pole. So magnetic pole will either attract or repel another magnetic pole depending on whether the other magnetic pole is of same sign or different sign or same type or different type. So north pole if you bring two north poles together they will repel each other. And if you bring a north pole and south pole together they will attract each other. But what is that force of attraction or repulsion or how that is calculated. So first thing that we need to remember over here is that ability of any magnetic pole to attract or repel any other magnetic pole is known as magnetic strength or polar magnetic strength or pole strength also it is called. And this is represented by a letter m. So in magnetism if you find a letter m that's related to m is nothing but pole strength and m is ability of pole of a magnet to attract or repel another magnetic pole. So that's what I have told you. So suppose one magnetic pole is here and its p is pole strength is m1 and second magnetic pole is here suppose its pole strength is m2 and distance between them is r. So force applied would be proportional to magnetic strength m1 and m2 and it would be antiproportional to the square of the distance between them. So I can write it as f is proportional to m1 m2 divided by r square. Now what is proportionality constant? Proportionality constant is nothing but mu naught divided by 4 pi and this m1 m2 divided by r square comes. So you always see whether it is the forces are always I mean we have seen it for gravitational force we have seen Kuhler Kuhler's law and now we are looking at this force due to force between two magnetic poles. It can be attractive force it can be repulsive force depending on the type of magnetic poles which are kept in vicinity. Now mu naught by 4 pi mu naught is equal to 4 pi into 10 to the power minus 7 and mu naught by 4 pi hence would be 10 to the power minus 7. So it's very easy to remember this this is nothing but 10 to the power minus 7 so that's how you calculate. Now what is magnetic flux or what is magnetic so before magnetic flux let me go to magnetic field. So how like we saw that in electric force also whenever two charges are kept in vicinity there would be some kind of electrical or electric force which is determined by Kuhler's law would be working between them. It can be a force of attraction or repulsion depending on the kind or kind of charge it is if same type of charges are there repulsion would be there and for opposite charges attraction would be there. But how do charges come to know that there are there is any other charge kept in its vicinity. So there is a concept of electric field line over there and the interaction of electric field line will determine what kind of forces applied. So in case of same kind of force for same kind of charges charges electric field lines overlap with each other they interact with each other and their interaction in such is in such a way that the forces are towards each other. So that is the force of attraction the repulsion repulsive force act because they have two if two same kind of charges have been kept their field lines will never cross each other as they will never cross each other. It shows that the two field lines will rebel each other hence the force of repulsion will work. Anywhere wherever there is a concept of force here also in magnetic field sorry in study of magnetism we will apply the concept of magnetic field. So what is magnetic field the field lines which are responsible for application of force on any particular pole by different pole. So magnetic force is influenced by magnetic field lines. So what is magnetic flux then so magnetic flux is nothing but number of field lines passing through any cross sectional number of field lines passing through any cross sectional area. So magnetic flux is given by Phi. So magnetic field is let me write few things over here is represented by B and flux is represented by Phi. So total number of field lines passing through any area. This is number of field lines passing through given area and magnetic field is flux per unit area. So per unit area whatever is the number of field lines passing that would be magnetic field. So that is how it is. Now you understand that suppose I define so try to understand B is defined like this. So suppose I take gravitational force due to earth so I write force on any particular object on earth as G mass of earth into mass of the object kept on earth divided by radius of the earth square. So now I write this as F MO into G. So this G is what G is the gravitational acceleration or gravitational force whatever you call that would be G. Similarly here F is written as B divided by M I mean suppose there are two poles and suppose this pole has strength M1 and this pole has strength M2. So on this pole if I want to find out the force it would be force B M1 into M2. And what is B M1 if I compare it with this formula mu naught by 4 pi M1 M2 divided by R square. So B M1 comes out to be equal to mu naught by 4 pi M1 by R square. So that's how you can correlate this. You can correlate Phi and you can correlate pole strength. Some question if you find correlation between Phi and pole strength so you should apply this formula you should always remember that pole strength B M1 is equal to mu naught by 4 pi M1 by R square. And B M1 is also equal to Phi divided by A. If you know Phi and A you can find out M1. Or if you know M1 and A you know you can find out Phi. So that is how it is. Now let's get into Austered experiment. So what did Austered told us that any current carrying wire produces magnetic field around it and it can detect a magnetic needle placed near it. So when Austered was doing an experiment there was a bare current carrying conductor and a compass kept near to it. When the current was passing through it the compass needles were moving when the current was stopped the compass needles were not moving. Hence it was determined that whenever there is a current carrying conductor so this is if this is current carrying conductor and a current is passing through it this current carrying conductor will produce magnetic field around it. Now what kind of magnetic field? So a straight current carrying conductor produces circular magnetic field around it. So it would be circular in nature. So something like this you will find out. So it is something like this. The second thing he told that intensity of the magnetic field is proportional to magnitude of the current passing through it. So that said magnetic fields set up acts at right angle to the direction of flow of current. So anywhere if you will find out the magnetic field direction it will be right angle to the flow of current. And direction of magnetic field depends on direction of flow of current also. So you can apply right hand rule here you can put your fingers in the direction of current. You can place your palm in the direction of the point where you want to find out the magnetic field. And the direction of the thumb will give you the direction of the magnetic field. I am again repeating it to find out magnetic field at any particular point. To find out yes, yes, yes, yes, Param you are absolutely right. So the direction of the magnetic field I am again repeating you can find it out by right hand rule. And right hand rules tells me that put your fingers in the direction of the current. The point where you want to find out the direction of the magnetic field and the thumb direction will give you the direction of the magnetic field. This is the right hand rule. So that's how you can find out. Now magnetic field due to a current. So remember this. So in your book there is only one formula given over here. So remember this formula. This formula is if a very long current carrying infinitely long. So they will never write infinitely long. They only write long current carrying conductor. So if a long current carrying conductor is there, the magnetic field due to a long current current carrying conductor B would be equal to mu naught by 2 pi I divided by D. What is the distance of the point where you want to find out the magnetic field? So suppose this is the point. So this is the perpendicular distance from this current carrying conductor. So distance of the point perpendicular distance of the point from the current carrying conductor would be known as would be known as the magnitude of the magnetic field. Now try to understand. I'm giving you a question. The question is so the question is so there are two current carrying conductors. One is like this and this is I1 and one is like this. This is I2 and distance between them is 15 centimeters. And here is a point which is 5 centimeter from here. At this particular point P you have to find out resulted magnetic field. Okay. What is the answer guys? I have an I2 I have already given to you. No, I'm not given I1 I2. Okay. I'm sorry. I1 I2. So just replace it in the value. I1 is equal to 15 amperes and I2 is equal to 10 amperes. Guys, after this question I will have to wrap up the session. So the rest of the things, rest of the magnetism portion perhaps before your examination just remind me I take that class and finish it off. So whenever you do your examination one or two days before it, I will take a class and I will finish it off. So don't worry about it. Roshil just remind me Roshil and Param. So now look at here guys the answer would be it is B1 plus B2. So look at here when I put my fingers in this direction and palm in this direction. So force would be in this direction. And when I put my fingers in this direction and palm again in this direction again the force would be in this direction or magnetic field would be in the same direction. So magnetic total resultant magnetic field B would be equal to B1 plus B2 and B1 is equal to mu naught by 2 pi I is 15 divided by 5 centimeters. So I will convert it into 5 into 10 to the power minus 2 and this would be mu naught by 2 pi I is 10 divided by this will be 10 centimeters or 10 into 10 to the power minus 2. So mu naught by 2 pi is 2 into 10 to the power minus 7 into 3 into 10 to the power 2 plus this will be 2 into 10 to the power minus 7 into this 10 10 will go this will become 10 to the power minus 2 only. So this is this is 6 into 10 to the power minus 5 plus 2 into 10 to the power minus 5. So this comes out to be nothing but 8 into 10 to the power minus 5. So sorry or 80 into 10 to the power minus 6. So this becomes 80 micro Tesla. So that's your answer. So your answer would be 80 micro Tesla. How many of you got it? How many of you got the answer? Okay. So guys I'm wrapping up the session. The rest of the topics I will revise it in the next class. So the topics remaining today are AC and DC generators and electromagnetic induction. So one hour class one one and a half hours of class would be sufficient enough for that. So I shall take that later. So what I'll do is that I'll cover that. I'll cover those topics in different classes. So just remind me before that. Okay. So thank you so much guys. Okay. No issues. Thank you so much all of you. I shall be wrapping up the session now.