Hmm, quantum physics?

[Tachyon]

Well, incase you know the path difference of the waves, Δl = k * λ.

Other than that, the k simply describes which maximum of the intensity of reflected light (or in this case neutrons) you're looking at. For different values (positive integers) of k you get different sin(ϑ), which all belong to a particle of the same wavelength - Means: Depending on which maximum (intensity - for reflected neutrons, intensity of the neutrons in a place, as diagram y=intensity, x=angle) you're looking at you will get 1,2,3 for the 1st, 2nd, 3rd, maximum.

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