Waves double-slit diffraction

The interference of microscopic particles leads to a diffraction pattern with deviations with respect to the mere sum of the individual probabilities. The two events are no longer independent. If we wish to state in advance where the next particle will appear, we are unable to do so. The best we can do is to say that the next particle is more likely to strike in one area than another. A limit to our knowledge, associated with the wave-matter duality, becomes apparent. In the double-slit experiment, we may know the momentum of each particle but we do not know an5 hing about the way the particles traverse the slits. Alternatively, we could think of an experiment that would enable us to decide through which slit the particle has passed, but then the experiment would be substantially different and the particles would arrive at the screen with different distributions. In particular, the two slits would become distinguishable and independent events would occur. No interference would be detected, that is, the wave nature of the particle would be absent. In such an experiment, in order to obtain information about the particle position just beyond the slits, we would change its momentum in an unknown way. Indeed, recent experiments have shown that interference can be made to disappear and reappear in a quantum eraser (ref. 6 and references therein). 

As a simple example, a diatomic molecule such as N2 with a certain N-N distance could act as a diffraction object with two scattering centers. The situation is very much analogous to a double-slit experiment with visible light. Those parts of the two spherical waves that emerge from the two sUts (or the two atoms) in phase lead to constructive interference. The diffraction angle G under which the conditions... 

Diffraction geometry for constructively interfering waves emerging from a double slit. 

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