Answer
$5.4\times10^{-7}m$
Work Step by Step
For constructive interference to occur, the path difference is a whole multiple of the wavelength. This is stated in equation 24–2a.
$$d sin \theta = m \lambda $$
Apply this to the fifth order bright fringe.
$$\lambda=\frac{ d sin \theta }{m}=\frac{(1.8\times10^{-5}m) sin 8.6^{\circ}}{5}$$
$$\lambda=5.4\times10^{-7}m$$
