 Okay, so now the chapter. So if there is a neuron in this part, okay, this is an experiment which has, you know, commonly discussed in detail. Let's call them S1 and S2. Okay, these two slits is on this plane. Okay, so light will come from here. This screen and this is D. And this is D, there is a screen. And we observe the things that happens on the screen. Okay, now if light is a particle, then what are the things you can expect in spot and here right spot. Okay, and here should be what? Dark spot. Fine. This is how it should be. But what turns out is that this is the brightness. It turns out that this is what is expected out of it. It is a small size. It's a small size. It's not fixed. Okay, so here you observe complete brightness in this experiment. This distance is small d. Distance between the slits is small d. And one more thing, very large compared to small d. Because it's an experiment. D has to be, capital D has to be very large compared to small d. Okay. Any doubt on S1 and S2? It is a point source difference. So these two ways, then you are going to see the image. Similarly, you have this screen over here. So whatever happens on the screen, you are going to see it and observe it. Fine. So like this, the situation is very nice. Now some of the difference between S1 and S2 when they have started. These are two coherent sources with zero part difference. How do you create such source to coherent sources with zero part difference? So it will generate ways and these two ways for reaching S1 and S2. So there is no part difference. And since it is coming, so assume that this distance is small d and capital D between this way and that way. Find out and this will be d. So what is d1 minus d2? Let's say l1 and l2. That is better. Find out l1 minus l2. How much? Whole square.