Waves that pass through the gap therefore change direction. If at any point of spacetime some waves of some kind, coming from wherever, will meet, they will interfere in some way or another depending on their parameters.We can see in this figure that these wave fronts become longer as they move away from the gap. Interfering waves don't need to be spherical or originate from the same source. Two such pulses in close proximity (close here depends on the spectrometer resolution) will also create interference fringes in the measured spectrum. This not only is seen in an experiment with two slits in space, but also you can use it to explain forming of ultrashort pulses through constructive interference of waves in some points in time and destructive in other. They can have different amplitudes, frequencies or phases, and it will influence how the superposed final wave (a sum of all the interfering waves amplitudes) will look like. On the other hand, interference is a phenomenon resulting from a superposition of waves. Nevertheless equations describing those two phenomena are very similar and for example are leading to the notion of solitons which happen both in space and in time through balancing of dispersion/diffraction with nonlinearity. Diffraction happens because directions of the k-vector spectrum differ, dispersion happens because phase velocity of each frequency differs. However, the causes of these two effects, diffraction and dispersion are a bit different. It makes a light pulse spread in temporal position domain during propagation, due to it having multiple frequencies (in temporal frequency domain) forming it. Similar effect to diffraction, which happens in space, is dispersion, which happens in time. It is related to uncertainty principle, because having an aperture confines light in spatial position domain therefore broadening its spatial frequency domain. You can think of it like a bunch of photons having a spread of momenta. It is due to the fact that the beam of light has some k-vector spectrum that has some finite width. $_2$Ĭredits: $_1$ Feynman Lectures on Physics $_2$Optics-Ajoy Ghatak.ĭiffraction is spreading of the beam of light as it goes through aperture or is emitted from a finite area source. We should point out that there is not much of a difference between the phenomenon of interference and diffraction, indeed, interference corresponds to the situation when we consider the superposition of waves coming out from a number of point sources and diffraction corresponds to the situation when we consider waves coming out from an area sources like a circular or rectangular aperture or even a large number of rectangular apertures (like the diffraction grating). To be more explicit read this passage from Ajoy Ghatak: The best we can do is, roughly speaking, is to say that when there are only a few sources, say two interference sources, then the result is usually called interference, but if there is a large number of them, it seems that the word diffraction is more often used.$_1$ It is just a quest of usage, and there is no specific, important physical difference between them. No one has ever been able to define the difference between interference and diffraction satisfactorily. Feynman has come from heaven to answer your question! Listen to him:
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