Young's double slit experiment with the source being a slit

[Cocoleia]

Homework Statement

Here i am asked to find the phase difference.

Homework Equations

The Attempt at a Solution

I know that usually the equation is mλ=asinΘ

 

[Cutter Ketch]

I assume you mean the difference in phase at the two slits.

Phase is where you are in the wave oscillation. Well, that is changing all the time, so usually we mean relative phase. If this point in the wave is at one point in the oscillation, then we know this point, say a half a wave further along in the propagation direction is always going to be a half a wave behind in its phase regardless of what moment in time we look.

See how I related distance ("half a wave further along in the propagation direction") to phase ("half a wave behind in its phase"). Does that give you any ideas?

You have two paths starting at the one slit and going to each of the two slits. You know those two paths have the same phase starting at the single slit.