 Let us generalize the formula first, general formula of the apparent frequency is this n is equal to C plus minus velocity of observer divided by C minus plus velocity of source times n naught. So, if source comes towards the observer it will be C minus V s in the denominator, a wave it will be C plus V s. So, do not remember it like that remembering the way which we have discussed just now will be much better. What we have remembered velocity of source in a denominator, velocity of observer in the numerator, frequency should go up if they are approaching, frequency should go down if they are going up, it might happen that one is approaching other is going away. For example, this is source which is going with let us say V 1 velocity, this is observer going with velocity V 2, speed of sound is C, original frequency n naught. So, what is the apparent frequency? C minus V 1 by C minus V 2. C minus V 2. Now, who is observer this is observer, observer will come in the numerator C plus V 2 minus V 2, because of this frequency should decrease C minus V 2 divided by C minus V 1, because of this also frequency should increase and it is in the denominator. So, you should decrease the denominator C minus V 2 times n naught. So, this is n. So, this is your school level stuff related to top loss of it. See, source is V 1 velocity, source is going towards the observer, frequency should increase because of V 1, it comes in a denominator. So, denominator will decrease, so that n naught increases, that is a minus here and it will be minus in the numerator also, because of that V 2 which is observer frequency should decrease. So, you will decrease the numerator, so that it decreases and you will decrease the numerator denominator also, so that it increases. So, it tries to compensate each other. Now, there are various scenarios, we are going away from the school, write down scenario 1, if wind is blowing, but it will affect, will it affect the speed of sound, it will affect the speed of sound, because sound is travelling inside the air only and air starts moving, are you getting it? So, the speed of sound will be what? C plus V w or C minus V w? Depends of the direction, depends on direction. So, if wind is blowing like this and sound is also going like that, the speed of sound it will take as C plus V w, if wind is like this, sound is like that C minus V w. So, effect of wind is it changes the velocity of sound, because that is a medium itself, getting it? So, instead of C, you have to write that in all the formulas that you have written, I doubt, can the source be observer? How should I write it? Sir, can you say when there is an echo of the source? I give an example, the dog barking, he was barking and he is hearing his own sound, I thought that a dog is that side, so he keep on barking. So, that thing can happen, when I am speaking I am able to hear myself also. So, source can be observer, there are many such scenarios write down, effect of reflection. So, when reflection happens, reflecting surface acts as observer initially. So, what will happen? First you take reflecting surface as if it is observer, can reflecting surface move with some velocity? It can move, so assume as if observer is moving. So, completely treat that reflecting surface as if it is observer, find out the frequency, which that observer as a reflecting wall will observe, then it will reflect the same frequency back, now it will become the source, and you have to account for the velocity now also, then now that will be velocity of source, reflect the same apparent frequency, now it will become source, getting it now it becomes source, observer could be the final observer. Sir, what will become the square being subjective? We will talk about that, we will take examples, that is all. So, we will take one example, I will put each and everything in that set, any doubts here? No doubts, okay, okay, this is wall, wall is moving with certain velocity, let us say this is v1, okay, this is vehicle, someone, this is param, this is param, param is moving towards the wall with vb, okay, wind is blowing in this direction, vw, speed of sound in stay air, speed of sound in stay air is c, he is shouting, and he is shouting with the frequency of n0, okay, the sound get reflected off from the wall, what frequency he will hear after reflection from the wall, it is just two step solution, it is not very complicated. So, step number one is what, wall is the observer, you can refer the equation formulas. Wall is the, step number one, wall is the observer, param is the source, right, speed of sound is what, what will take speed of sound as? C plus vw, now both are approaching, they will contribute to increase the frequency, so n will be equal to speed of sound plus velocity of observer, who is observer, v1 divided by c plus vw minus v2, this n0, how many of you got this, okay, so this is the frequency the wall will receive as a source, what frequency wall will reflect back, same, this frequency n will be reflected by the wall, but now wall is the, wall is source, roll reversal, the final frequency guys keep quiet, now what will take speed of sound as, this way sound is coming down like that, so c minus, who is, who is source, wall is source, this is observer, so plus v2 or vp divided by c minus v2, this is the frequency minus vw minus v1, that times n, that times n and what is n, c plus vw plus v1 divided by c plus vw minus v2, how many of you got this, fine, so this is got the frequency and the chapter is over, syllabus is over, see it is there, I know it is there, see, listen, I want you to learn that when you solve numerical, okay, it will be a part of a numerical, okay, there is a concept of intensity of the wave also, there is a derivation of that as well, okay, solve the numericals corresponding to the syllabus of the NCRT first, okay, no point telling you the extended topics and you are not even doing the basics first, okay, if you are able to solve the basics, then we can do the basics first, okay, okay, if you are able to solve the basics, then we can extend it, I mean it will hardly take 15-20 minutes to do the extended topics, but I don't want to keep dumping a lot of information, read it, read it, read it, okay, fine, give me the centre module if you have centre module for waves, you mark it right now, okay, if you see the past archive, they don't go beyond NCRT curriculum, we'll do some advanced level questions, okay guys, this is J-Advance 2017, okay, okay, look, stationary source emits the sound of frequency, I'm not going to write each and everything, okay, listen to each and every word, a stationary source emits this frequency, the sound is reflected by a large car approaching the source with a speed of the velocity of car is given 2 meter per second, so stationary source emits this frequency, the sound is getting reflected by the large car approaching the source with this velocity, okay, the reflected signal received by the source and so proposed with the original, listen, the reflected signal is received by the source and so proposed with the original, what will be the beat frequency of the resulting signal? Beat frequency, the velocity of sound is given as 330 meter per second, it's an integer time, all of you got 6? No, you're not getting, approaching the observer which is stationary, you should be drawing the diagram first, okay, car is telling with 2 meter per second, so the frequency the wall will receive is, wall is observer right now, so 330 minus, sorry, plus 2 divided by 330 into 492 is received, this will be transmitted back and now what this guy will receive is speed of sound, reflection, this times N1 and N1 is this, it's much simpler than what we have done, and the beat frequency is what? Original frequency minus N2, what is this, whatever it is, this is mod of that, it comes out to be 6, they are very big word problems, so I can't