Hi all "The Huygens-principle recognizes that each point of an advancing wave front is in fact the center of a fresh disturbance and the source of a new train of waves; and that the advancing wave as a whole may be regarded as the sum of all the secondary waves arising from points in the medium already traversed." (source: Wikipedia) Applying this principle would mean that that the light coming from a source (eg. a laser) quickly spreads out even behind the source. One would also expect to see inteference-patterns on a screen placed behind and not infront of a double-slit. Obviously this is not the case (or is it??). So is there any extended huygens-principle which describes at which angle the elementary waves emit or that gives some other explination?? Thanks in advance!