Final answer:
When the indicator on a lens clock changes when the lens is rotated, it indicates that the lens surface is not flat.
Step-by-step explanation:
The change in the indicator on a lens clock when the lens is rotated indicates that the lens surface is not flat.
If the lens surface were flat, the indicator on the lens clock would not change when the lens is rotated.
Instead, the change in the indicator suggests that the lens surface is curved or has an irregular shape.
This curvature or irregularity affects the path of light passing through the lens, causing the ray of light to bend or refract and resulting in the change in the lens clock indicator.