approximately equal to 1). Thus, combining these phase differences with
λ
= 0.600
µ
m,
we have
(
)
(
)
(
)
(
)
2
1
2
2
1
1
3.50 m
4.00 m
3.50 m
1
1.60
1.40
1
1.40
0.600 m
0.600 m
0.833.
L
L
L
n
n
n
µ
µ
µ
λ
λ
µ
µ
−
−
−
+
−
=
−
+
−
=
(b) Since the answer in part (a) is closer to an integer than to a half-integer, the
interference is more nearly constructive than destructive.
13. (a) We choose a horizontal
x
axis with its origin at the left edge of the plastic.
Between
x
= 0 and
x = L
2
the phase difference is that given by Eq. 35-11 (with
L
in that
equation replaced with
L
2
). Between
x = L
2
and
x = L
1
the phase difference is given by an
expression similar to Eq. 35-11 but with
L
replaced with
L
1
–
L
2
and
n
2
replaced with 1
(since the top ray in Fig. 35-37 is now traveling through air, which has index of refraction

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