Hello! I'm new here, and this is my first post. I hope I have not breached any rules, but here's a rather strange (in my limited intelligence) question I encountered. I shall try to explain the question because I am unable to reproduce the diagram. Two identical narrow slits S1 and S2 are illuminated by light of wavelength (lambda) from a point source P. [diagram = So, P (on the left) is connected to the line marked S1 and S2 by two lines l1 and l2. l1 is longer than l2. A point Q on the right of the line S1S2 is connected to S1 by l3 and S2 by l4, l3being longer than l4] The light is allowed to fall on a screen (at Q), and if m is a positive integer, the condition for destructive interference at Q is that... Thanks for the help! EDIT: Crap, I just realised this is in the wrong thread... I really apologise for bungling up my first post and the inconvenience I have caused.