Answer
580 nm, yellow light.
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 first order fringe on a screen a distance $\mathcal{l}$ away occurs at a distance of $\frac{\lambda \mathcal{l}}{d}$ from the centerline.
Here, that distance is 0.0029 m. Solve for the wavelength.
$$0.0029m = \frac{\lambda(2.5m)}{0.00050m}$$
$$\lambda=5.8\times10^{-7}m =580 nm$$
From figure 24–12, we see that the light is yellow.
