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Young’s slits

Size and spacing of slits for Young’s Fringes – the theory

The amount of light reaching the fringe pattern is determined by the width of each slit of the pair. The wider each slit is, the more light. 

But the width of the patch over which the fringe pattern is spread (by diffraction from each slit) varies inversely as the width of the individual slits. Therefore, wide individual slits give more light concentrated into a narrower region, making the fringes much brighter. On the other hand, the narrower that bright patch formed by diffraction, the fewer fringes there are visible – and students need to see several dark and bright fringes to be convinced. The closer the two slits are together centre-to-centre, the greater the spacing from fringe to fringe, and the easier the fringes are for students to see. 
Therefore, we should aim at using a double slit with the two slits each as wide as possible and with as small a separation between them as possible. 
The widest slits allowable for a reasonable picture of fringes seem to be slits of width x whose centres are a distance 2x apart. That is, two slits of width x with an opaque region of width xbetween them. That arrangement will give three bright fringes with two dark fringes between them, and little illumination in regions beyond that. If the slits are themselves too wide or are too far apart, the central maxima of the diffraction patterns may not overlap and no fringes will be seen. 
Very thin slits are ideal for making a broad display of many fringes: but that display would have to be observed with a magnifying glass or photographed, because the fringes are both fine and faint — then the cogent simplicity is lost.