In a double-slit experiment the slit width a equals one-half of the center-to-center separation d between the slits. Monochromatic light of wavelength λ produces a series of interference maxima on a distant screen, but the m=2 bright fringe is not observed. Which of the following best explains the absence of this fringe?
(a) The m=2 bright fringe is too close to the central maximum to be resolved by the detector.
(b) At the position of the m=2 fringe the path difference equals 2.5λ, leading to incomplete constructive interference and hence little or no intensity.
(c) For a=21d, the m=2 interference maximum occurs at the same angle as the first minimum of the single-slit diffraction pattern, so its intensity is effectively zero.
(d) Because each slit is very narrow, the diffraction envelope is so broad that higher-order maxima like m=2 overlap and blend into the background, so the fringe is not observed.