Youngs double slit

a while ago in school physics, my class (all 3 of us) did youngs double slit with a laser, and the board. when the teacher was explaining to us how it worked, the varying amounts of interference along the line. he said that the interference was caused by the waves being out of sync due to the fact that they have to travel different distances. it occured to me that there must be a microscopic amount of time at which the diffracted light creates a solid line, as only the light that has the shorter distance to travel has reached the screen, the light with further to travel, would reach it almost immediately after, creating the interference pattern, but there must still be a point, at which the makes an unbroken line.

Hmm... Sounds interesting but I am not familar with the experiment ( they didn't have laser pointers back then) so you would have to give a better description of the experiment?