Answer
$6.4\times10^{-7}m$
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 spacing between successive fringes on a screen a distance $\mathcal{l}$ away is $\Delta x = \frac{\lambda \mathcal{l}}{d}$.
By using a ruler on Figure 24–9a, we measure that the spacing $\Delta x$ is about 1.4 mm. Solve for the wavelength.
$$\lambda=\frac{d\Delta x}{\mathcal{l}}=\frac{(0.17\times10^{-3}m)(0.0014m)}{0.37m}$$
$$\lambda=6.4\times10^{-7}m$$