Final answer:
The difficulty in seeing motorcycles, besides their smaller size, is due to visual acuity and the wave nature of light.
Step-by-step explanation:
The difficulty in seeing motorcycles, besides their smaller size, is due to a phenomenon called visual acuity. Our eyes have limitations in perceiving small objects, especially when they are far away. One of the main reasons for this is the wave nature of light. When light interacts with small objects, it diffracts and spreads out, making it harder for our eyes to distinguish fine details.