The ruler is thin by the edges so that the markings can be easily compared with whatever is being measured. It is thick in the centre so that it does not snap too easily.
The edges of a convex lens are thicker than the middle.
the centimetre ruler
thicker at the edges that in the middle
thicker, thinner
thicker at the edges that in the middle
arteries.
Concave
convex
no
In a convex lens, the middle is thicker than the outer rim. In a concave lens, the outside rim is thicker.
CONCAVE lens.
Concave. The opposite- thick in middle, thin at edges- is convex.