Figure 7: Action of a lens in a dual-slit setup—slits 1 and 2 create a two-slit image 1 and 2 , denoted are the focal plane of the lens 𝑓 = 2 . 2 5 m and the image plane of the lens 2 𝑓 . The wire grid that could be used to verify the existence of an interference pattern in the coherent setup when both slits are open is not shown because this is analytically true statement (provable as theorem) and we do not need to verify the presence of interference experimentally in order to know that the theorem is true. If we accept the standard QM postulates, then we must accept the interference as true statement as well, even though we have not measured it. Plotted are the probability density functions at the level of the optical axis ( 𝑦 = 0 ) for photon with 𝜆 = 6 5 0 nm passing through square slits with side of 250 μm separated by distance of 2 mm.