Why the effect of diffraction gets weaker as the distance between slits increases? I was studying the light interference and I heard that in order to get a clear and fancy result the distance between slits has to be comparable to the wavelength not much bigger and I can't understand why so if anyone can help me understanding this I'll be grateful to him.
 A: For a given wavelength, the greater the distance between the slits, the smaller the angles at which the difference between the two paths is equal to a multiple of half wavelengths, the closer the intervals between maxima and minima on the screen, the less distinct the interference pattern.
For instance, the angles associated with maxima could be found from this formula: 
$\sin \theta = \frac {m\lambda} d$
From here, we can see that a greater distance between the slits, d, translates into smaller angles between the maxima, which, for a given distance to the screen, translates to a denser, less distinct interference pattern.
A: @VF is right that the distance between fringes depends inversely on the distance between slits, meaning that it could be hard to resolve fringes that are close together when slits are far apart. But even if you have the ability to resolve closely-spaced fringes, another aspect you need to consider is spatial coherence.  The light at each slit needs to be mutually coherent to get interference, and as the slits get farther apart, the mutual coherence will likely diminish (unless you are using a nice laser source).  This reduction in coherence will wash out the fringes.  The solution for low-coherence light is to use slits spaced on the order of the wavelength or less, where there is always some mutual coherence.
