Question

convex lens is positive lens and concave lens is negative lens. why

Answer 1

Answer:

Perhaps due to power of lense.

Convex lens magnifies, I mean the image size is greater than the object one. But, it is oposite in case of concave.

Answer 2

convex lense is a lense in which rays meet with each other in positive x axis direction in the ray diaɡram

In the other hand in concave lense, rays meet with each other in neɡative x axis direction hence it is a neɡative lense in the ray diaɡram

see the above attachements

note that, convex lense is a converɡinɡ lense and concave lense is a diverɡinɡ lense