Here's the problem: Two light sources can be adjusted to emit monochromatic light of any visible wavelength. The two wources are coherent, 1.72e-6 m apart, and in line with an observer, so that one source is 1.72e-6m farther from the observer than the other. What is the longest of the visible wavelengths (400 to 700 nm) at which the observer will see the brightest light, owing to constructive interference? I read the section in the book that problem came from, but the only equation I can find is r2-r1=m*lambda (constructive interference, sources in phase). How do I apply this equation to this problem? This is probably my last physics question, since our final is Monday. I just want to thank everyone on this board. You have been so helpful to me...I don't know how I would have gotten my homework done without your help!!