Final answer:
The number in the name of flat top or straight top bifocals indicates the positive optical power of the lens for farsightedness correction.
Step-by-step explanation:
For flat top, also known as straight top bifocals, the number in the name indicates the positive optical power of the lens segment added to correct farsightedness (presbyopia). This additional power helps in converging light rays to assist with near vision tasks.
The lenses in these glasses will be thickest in the center, highlighting their convex nature. On the other hand, lenses for nearsighted people are thinnest at the center and have a negative power, indicating they are diverging (concave) lenses.