Answer
$d=1.5\times10^{-4}m $
Work Step by Step
The spacing on the screen between bright fringes is addressed in Example 24-1.
For light shining through 2 slits separated by a distance d, the third order fringe on a screen a distance $\mathcal{l}$ away occurs at a distance of $\frac{3 \lambda \mathcal{l}}{d}$ from the centerline.
The distance is given as 38 mm. Solve for the slit separation.
$$0.038m = \frac{3 (680\times10^{-9}m)(2.8m)}{d}$$
$$d=1.5\times10^{-4}m $$
