adjective
"Convex" is an adjective.
It all depends on whether you are inside or outside of the sphere.
The convex lens are the converging lens. The periphery of the convex lens is thinner & the center part is thicker. In other words, the convex lens are bulging at the center & thinner at the periphery.
In a convex figure, if you pick any two points, the points between them are also a part of the figure.
The 'face' of the weld may be slightly convex, flat, or slightly concave.
It depends whether it's a concave or convex lens. If it's a convex lens - the thickest part is at the centre. With a concave lens, the thickest part is around the edge.
The part of speech for this particular word is a noun.
part of speech
The part of speech for "answer" is a noun.
what part of speech is beneath
adverb
The part of speech for "explicit" is an adjective.