81. We adapt the result of problem 21. Now, the phase difference in radians is
2πt
λ
(n
2
− n
1
) = 2mπ .
The problem implies m = 5, so the thickness is
t =
mλ
n
2
− n
1
=
5(480 nm)
1.7
− 1.4
= 8.0
× 10
3
nm = 8.0 µm .
81. We adapt the result of problem 21. Now, the phase difference in radians is
2πt
λ
(n
2
− n
1
) = 2mπ .
The problem implies m = 5, so the thickness is
t =
mλ
n
2
− n
1
=
5(480 nm)
1.7
− 1.4
= 8.0
× 10
3
nm = 8.0 µm .