Why does narrower slit give a wider diffraction pattern in single slit diffraction? I was reading on this website and it said that one of the characteristic of single slit is narrower slit give a wider diffraction pattern but why?
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/sinslitd.html
 A: The property of light being collimated ("all going in one direction") is actually an interference property which requires the light to have a certain coherence in phase over its perpendicular extent. Light by its nature would rather spread out in all directions.
To provide this pattern that perpendicular extent has to have a certain characteristic length related to the wavelength of the light itself. When the slit gets narrower than the wavelength of light, this coherence becomes less and less possible and the wavefronts from this point which want to spread out in all directions can no longer be cancelled out by wavefronts from that point next to it, which want to spread out in all directions -- hence without this cancellation the main peak starts to widen.
There exists another way to view this variation of phase as the light's "perpendicular momentum", and forcing the light into a more and more rigid position spreads out its momentum by a sort of position-momentum uncertainty. In fact this effect was well-known for waves far before Heisenberg's uncertainty principle, and it is more appropriate to view the Heisenberg uncertainty principle as a rudimentary prediction of the de Broglie hypothesis that all material particles behave like waves with a momentum inversely proportional to their wavelength.
