Two in-phase sources of waves are separated by a distance of 4.00 m. These sources produce identical waves that have a wavelength of 5.00 m. On the line between them, there are two places at which the same type of interference occurs. Where are the places located? I already know the answers from this website http://lev.ccny.cuny.edu/~hmakse/TEACHING/204soln27.pdf [Broken] problem 3. However I cannot for the life of me understand the solution. I especially don't understand why d1 + d2 has to be less than 4. I have spent an hour on this problem and re-reading the explanation and my textbook. But it just doesn't click.