 So, when we discuss about the pathways, assume that light is a particle and if it is a particle, you know straight line its path will be well defined, what is the path of the light? So, you will be able to see the path as a straight line and until it hits an object, suppose there is a reflection, its path won't be changed. So, it will travel in a straight line. So, we have discussed about reflection as well as refraction when it comes to rare objects. Fine? A few of you can sit there, two of you can sit there. You do too, you do. So that I don't only look to the other side. So, now we have discussed initially itself that light can behave like, you will be showing multiple kinds of behavior, but light has a wave also. Now is the chance to study the light when it behaves like. Now, what is a wave? What it is? Simple. It is a disturbance that travels in just a disturbance. Mechanical waves travel in disturbance. Have we learned about it? So, in class 12th, light is one of the disturbance. What is a wave? Wave. So, what it is, we have learned about transverse waves. In waves chapter, in class 11th, still holds good here also. In the chapter, when you studied the waves, was waves only because one may not just study. We have studied few general things which is true for all the few specific things which are true for mechanical waves. So, general things still hold good. When you don't start, midway will be faster. Fine. So, you know, neutral waves, it's all the difference. Fine. And it's horizontal. So, this bracket... This is the only way to study for transverse waves, the hindu. Anyways, so, that is why we need to understand the wave concept. First of all, there is to check the validity of what Newton is saying. Second, nobody will challenge Newton. Fine. So, but then, that time also, there are few scientists who are delivering the idea of line which Newton would not explain. Okay. That we are going to take up a little bit later. Fine. So, my point is that, it took years in the concept of wave optics. Okay. Because of optics, so-called scientists, they were fascinated with the particle nature of the light. Fine. It took a lot of time to take shape. Fine. One of the forefathers, fine, he has actually conceptualized everything. Fine. So, we'll start this chapter with Huygens principle. Right. Write down Huygens principle. Is the chapter done in school or? It's going on. It's finished. It's finished. Good. So, Huygens principle, fine. See, always. So, Huygens is the one principle which connects that. Fine. So, this becomes, so to say, for this chapter. All right. So, we need to understand it properly. But then, before I understand it, I want to solve it. Fine. So, first tell me, you have any shapes, the circle. Why do you say it is circle? Because you are able to see the circle. And why are you able to see the circle? How about now? What is phase? What is in phase B? In phase B. So, which are the ones which are in phase one and four, probably. You can say one is practical. Okay. So, similarly, points in phase and that emits light in every direction. What kind of, what if it is a point source? Spherical. Spherical. Okay. So, these things, each point of the wave front is a source of secondary disturbance. Of secondary disturbance. Spread out. Spread out. That each point will have its own. So, we have five points that will behave like a source of secondary disturbance. Or, I just draw one sphere. So, this is a source. And this one, same distance will be covered. So, this will be. So, it will be.