Nλ=d sinθ 2dn=(m+0.5)λ and nλ 2dn=mλ and (n+0.5)λ.
N generally stands for refractive index, but here some places indicate the degree of diffraction and interference, so it is better to change it to m.
Mλ=d sinθ: grating equation
2dn=(m+0.5)λ and mλ, where 2dn is the optical path difference of thin film interference, (m+0.5)λ represents dark fringe and mλ represents bright fringe.