Is passing of light through small opening required for interference.? Is it necessary that light passes through a small opening to obtain interference or can interference happen even when light did not pass through a small opening? My understanding of interference is that it happens due to the combining of two waves. I think that it can happen without light having to pass through a small opening.
 A: Interferometers are another way to get interference without small structures.
A: Light needs to coherent for it to produce an interference pattern. So, if one has two mutually coherent sources, then they will interfere without having to pass through a small opening. However, one can use a small opening to produce coherent light is the source is not coherent. For more detail on this aspect, see the Van Cittert-Zernike theorem. 
A: It's not at all necessary. Interference is basically interaction between two(or more) waves which leads to a resultant wave with a different or same amplitude as one of the waves.
I'm assuming that you've encountered this 'openings' concept during the 'Young's Double Slit Experiment' in that case we have such restrictions (small opening, coherent sources) mainly to ensure that we are able to see the interference pattern.


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*Small Opening: So that we avoid diffraction, again diffraction and interference are arguable terms but let's consider that diffraction happens by interaction of multiple(more than a few) sources are considered but interference when only a few sources are considered(it deserves an answer of it's own and cannot be explained in entirety)

*Coherent Sources: Again it's essential for us to be able to see consistent interference pattern, if the sources are not coherent their phase difference will be continuously changing and thus we won't be able to observe the interference pattern.
Strictly speaking non-coherent sources also do interfere and such interference is called heterodyne interference, the only thing is we cannot see such interferences with our naked eye so we prefer coherent sources.

