In one version of the double slit experiment, the experimenters placed a convex lens far away from the two slits. After the individual photons passed through the two slits they then travel to the the lens, then after that they hit the back wall. However the detector screen at the back wall is placed after the focal point of the lens. And the detector screen shows that the photons hit the detector screen in a particle fashion corresponding to the two slits. My question is, what happens the individual photons after they pass the focal point after the lens that destroys the interference pattern and makes a particle pattern?
The interference pattern is not destroyed by the lens or the focal point, because in this case it never existed. The moment the lens is placed between the slits and the detector, all particles traveling through the slits will only behave like particles passing through a slit then a mirror and then hitting one of 2 points on the detector. Designing the experiment in any way that allows the path of the particles as they are travelling between the slits and detector to be known is enough to prevent the interference pattern from showing. The particles only behave like waves when you are not looking at their path.
The link below explains it clearly. Paragraph 3 talks about the experiment with the convex lens, but the whole paper is worth a read.