Constructive: 2t = (m + 1/2)(λ/n) in one special case!

Destructive: 2t = m (λ/n) in one special case!
Here m is any positive integer.
These equations are for the case that nfilm is greater than nair. Instead of “remembering a formula,” which will be inappropriate half the time, it is vital to work out the path and phase differences from scratch in each individual example.