Answer
$258nm$
Work Step by Step
We can find the required minimum thickness of the soap film as follows:
$t_{min}=\frac{m(\lambda)}{2n}$
We plug in the known values to obtain:
$t_{min}=\frac{(1(670\times 10^{-9}))}{2(1.30)}$
$t_{min}=257.69\times 10^{-9}m=258nm$
