This is question number 7P in chapter 27 in Cutnell and Johnson's Physics 8th Edition. A very good solution can be found there, there is just one part of the solution that I don't understand.

Two parallel slits are illuminated by light composed of two wavelengths, one of which is 645 nm. On a viewing screen, the light whose wavelength is known produces its third dark fringe at the same place where hte light whose wavelength is unknown produces its fourth-order bright fringe. The fringes are counted relative to the central or zeroth order bright fringe. What is the unknown wavelength?

Here is a link to the solution http://www.chegg.com/homework-help/physics-8th-edition-chapter-27-problem-7p-solution-9780470223550

Can anyone explain to me why m=2 is used for the 3rd order fringe? Using m=3 produces the wrong answer, I tried it.


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