58. The angle of incidence on the reflection planes is θ = 63.8
◦
−45.0
◦
= 18.8
◦
, and the plane-plane separation
is d = a
0
/
√
2. Thus, using 2d sin θ = λ, we get
a
0
=
√
2d =
√
2λ
2 sin θ
=
0.260 nm
√
2 sin 18.8
◦
= 0.570 nm .