Answer
The tiny mirror needs to be moved away another $~~310~nm$
Work Step by Step
In part (a), we found that the smallest value of $L$ is $155~nm$
The path length difference is $2L$
To be exactly out of phase, the path length difference is $~~(\frac{\lambda}{2}+m~\lambda)$, where $m$ is some integer
We can find the second smallest value of $L$:
$2L = \frac{\lambda}{2}+\lambda$
$L = \frac{3\lambda}{4}$
$L = \frac{(3)(620~nm)}{4}$
$L = 465~nm$
Therefore, the tiny mirror needs to be moved away another $~~310~nm$
