Final answer:
Felicity is observing the diffraction effect, which occurs when light waves bend around an obstacle or aperture and create a pattern of alternating bright and dark areas. In this case, the sun's rays are being diffracted by the watchtower, drawing attention to its borders and outlying features.
Step-by-step explanation:
Felicity is observing the diffraction effect. Diffraction occurs when light waves encounter an obstacle or aperture and bend around it, causing the light to spread out and create a pattern of alternating bright and dark areas. In this case, the sun's rays are being diffracted by the watchtower, drawing attention to its borders and outlying features. This phenomenon is commonly observed when light passes through small openings or around corners.