Final answer:
The light observed in the 'geometrical' shadow of a narrow slit is due to diffraction, a consequence of the bending of light as it passes through small openings, according to Huygens's principle.
Step-by-step explanation:
When a beam of light passes through a narrow slit, the light that is observed in the 'geometrical' shadow of the slit arrives by the process of diffraction. This occurrence can be explained using Huygens's principle, which suggests that every point of a wavefront can be considered as a point source of wavelets spreading out in all directions. When light, or any wave, passes through a small aperture or around an obstacle, it bends and spreads out, which is the defining characteristic of diffraction. In the context of a single slit, light from various points within the slit can interfere destructively or constructively, depending on the angle, causing a diffraction pattern to appear even though there is only one slit involved.