My book says that "One property of a converging lens is that it focuses all rays that are parallel to one another to the same point on its focal plane". But isnt that wrong? I mean, dont the rays have to be parallel to the central axis through the lens, not just to one another? In any case, my book is describing a situation where you have a convex lens between a double slit and screen, with the screen exactly focal length from lens, and light being shined through the slits. My book says that in this case, all of the rays that end up on the screen had to have been initially "parallel"..im assuming they mean parallel to central axis, but they dont say to what..but then I think they want to justify in the derivation of the interefence formulas the assumption made about the rays ending up at any point on the screen being parallel to each other...but I dont understand why all of the rays on the screen must have had initially been parallel...not all of the rays entering the slit are parallel to each other or to the central axis, and so they all hit the lens in different ways, and so some end up at the focal point and some dont, but they all end up on the screen, regardless if they were initially parallel or not.. I understand how interference and diffraction with single and multiple slits works, but I dont understand at all what my book is trying to say about the lens in the middle..Could someone please explain the significance of the situation? Is it somehow different from when you just have the slits and screen?