Answer: 12.50 μm
Explanation:
Usually, reflections from the top of the bottom plate have a ½λ phase change. But they do not possess it from the bottom of the top plate (striking air). So then,
t = ½ λ = ½ x 450*10^-9m = 225*10^-9 m
So that the Wedge angle will then be
sinθ = t/1.80*10^-3 m
sinθ = 225*10^-9 / 1.80*10^-3
sinθ = 1.25*10^-4
θ = 2.18*10^-4
For the whole wedge, the hypotenuse = 0.10m, i.e the vertical wire diameter = d
Sinθ = d / 0.10
1.25^-4 = d / 0.10
d = 0.10 x 1.25*10^-4
d = 1.25*10^-5m
d = 12.50^-6 m (12.50 μm)