Log-log plot showing the flash profiles caused by a spherical and transparent Pluto’s atmosphere, as derived from the 6 June 2020 occultation (Sicardy et al. 2021) and using the parameters listed in Table 2. Red curve: the flash caused by a point-like star and monochromatic wave, giving a finite diffraction peak and interference fringes (because Pluto’s atmosphere is dense) with spacing λ_P ≈ 1.2 m (Table 2). The flash has then the same radial dependence as the Poisson curve shown in red curve in Fig. 3; Black curve: the flash caused by a star with a diameter D_* = 0.5 km projected at Pluto’s distance (Eqs. 57 and 61); Blue curve: the flash obtained using the geometrical optics approximation (Eq. 6).