We know that to observe diffraction the width of the apertures in the grating must be of the order of the wave-length used. Would we observe diffraction if the apertures are significantly narrower than the order of wavelength used? Or what would be the difference? I think we would not observe diffraction at all. Because at the mentioned case, the aperture can be considered as a point source. But I have confusions and don't have clear-cut idea about why it should be like that.