Illustration of the ray geometry in spherical (upper half) and plane-parallel (lower half) symmetry. Gray regions correspond to the optically thick regions of the star. R_⋆ is the stellar radius, z the vertical coordinate, and r_top the – somewhat arbitrary – uppermost level in the spherical or planar atmosphere. Red arrows depict how straight light rays in spherical geometry get bent when interpreted in planar geometry. The blue arrow depicts a ray in planar geometry. The yellow circle marks the location where the inclination angle of rays in both geometries coincides. The green angle illustrates the inclination angle of a ray with respect to the vertical. “Core rays” are rays that penetrate into the optically thick part of a star.