"Convex" is an adjective.
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.