K-order dark fringe Sinθ 1 = kλ/a. 1 for single-slit diffraction dark fringe condition asθ= kλ. Similarly, k+ 1 order dark stripe sinθ 2 = (k+1) λ/a.2.

Subtract 1 from Equation 2 to get the angular interval between two adjacent dark lines: sinθ 2-sinθ1= λ/a.

As can be seen from this formula, the angular spacing between two adjacent dark lines is inversely proportional to the seam width a.

So the reason why the seam width becomes smaller and the stripe spacing becomes larger.