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m is the integer order of the diffraction maximum. When light from many slits in a grating interferes, a bright fringe occurs only when the extra distance light travels from adjacent slits equals an integer multiple of the wavelength, so the waves arrive in phase: d sin θ = m λ. The central bright spot is the zero order (m = 0), with first-order maxima at m = ±1, then ±2, and so on. Only integer values of m produce perfectly constructive interference across all slits, which is why they label the bright fringes.