B

Fig. 1. Young's slits in various situations. In each panel, the source is shown on the left, and on the right of the slit are shown the fringe patterns separately for each part of the source and then the added fringe pattern; (a). The basic two-slit pattern, showing fringes an angular distance X/d apart; (b). The effect of increasing the source size. An angular shift of the source position by 6 shifts the fringe patterns by 6 the other way. Since the patterns come from mutually incoherent sources, the intensity patterns add to give a pattern of reduced visibility; (c). When the size of the source reaches X/d, the fringes add to give zero visibility; (d) If the slit spacing is then reduced, the fringe spacing increases, and the same size of source is still able to give visible fringes: the source would need to be increased in size to X/dnew in order to wash out the fringes c d

Fig. 1. Young's slits in various situations. In each panel, the source is shown on the left, and on the right of the slit are shown the fringe patterns separately for each part of the source and then the added fringe pattern; (a). The basic two-slit pattern, showing fringes an angular distance X/d apart; (b). The effect of increasing the source size. An angular shift of the source position by 6 shifts the fringe patterns by 6 the other way. Since the patterns come from mutually incoherent sources, the intensity patterns add to give a pattern of reduced visibility; (c). When the size of the source reaches X/d, the fringes add to give zero visibility; (d) If the slit spacing is then reduced, the fringe spacing increases, and the same size of source is still able to give visible fringes: the source would need to be increased in size to X/dnew in order to wash out the fringes

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