Hi All, Just got to thinking about interference, if we have two sources (say a laser beam split in two), and introduce a phase shift between the two by making them travel different path lengths, then recombine them, we see fringes in the wavefront right. (interferometry) My problem is this, if we have two perfectly monochromatic and perfectly collimated (i.e. no divergence as they propagate out to infinity) beams, the fringes would not be visible right? as the transverse pattern occurs due to the fact that the beams are indeed slightly divergent. You would however see a spot of light whose brightness in proportional to the phase difference between the two beams. so the only reason we have interferometry is because we cant make perfect beams. Does this make sense? Thoughts please.