Sure - this seams to get asked a lot but I am not entirely getting my head around it. Why does the double slit provide perfectly vertical interference lines? I understand the premise of why this would happen in a 2d world - the usual wave analogy applies. However, in 3d it surprises me that the lines continue up and down in a straight line. I imagine the following; - two Long slots. - a textbook interference pattern is generated at the screen centre - this relates to light hitting the screen traveling perfectly normal to the screen, slots, and laser. - at every position up and down the full length of the slots the pattern occurs and we have our vertical interference strips on the screen. But what happens to say the bottom left and top right slot positions interference with each other? And then every other combination in between? Wouldn't this make the interference pattern have bent ends on every strip rather than being straight lines?