Upper panel : same as Fig. 7 but with a larger wavelength λ = 1 mm, hence a Fresnel scale λ_F = 50 km. The Fresnel fringes seen at the outer part of the shadow (|x| ≳ 1020 km) are separated by several tens of kilometers, while the Poisson fringes appearing at |x| ≲ 1020 km, have separations of about 2 km and are not resolved in the panel. The 50-km-wide box (magenta) shows the resolved Poisson fringes vanishing as the secondary image disappears behind the limb. The green curve is the difference between the blue curve (wave optics) and the gray curve (geometrical optics), displaced vertically by −0.15 for better viewing. Middle panel: 2D map based on the green curve of the upper panel, enhancing the difference between the Fresnel and Poisson fringes. Lower panel : enlargement of the middle panel, which reveals the fine striations caused by the Poisson fringes.