Answer
The phase difference is $~~1.25$
Work Step by Step
Note that the phase difference is $\frac{\lambda}{2}$ when $n = 1.4$ because the first minimum occurs when $n = 1.4$
We can find an expression for $L$:
$phase~difference = 1.4~L-L = \frac{\lambda}{2}$
$L = \frac{\lambda}{0.8}$
$L = 1.25~\lambda$
We can find the phase difference when $n = 2.0$:
$phase~difference = 2.0~L-L = L = 1.25~\lambda$
As a multiple of $\lambda$, the phase difference is $~~1.25$
