 See how the fringes will be formed. Suppose you are looking for this, touching the screen. What you see? Center. So if you look at the center, it will be black marker, but consider it as second. And so you can see that approximately these things are straight. So distance between these two, these two increase towards the right. No fixed thing like that. Don't create, don't start distance, the distance, small d. When these two ways start from slits, for this particular case. This is small d, the distance will be.