Step-by-step explanation:
The imaginary line passing through the center of the lens is called the principal axis of the lens.
If the incoming light rays comes parallel to the principal axis of a convex lens, then after refraction it will pass through the principal focus on the other side of the lens. All the rays meet at the focal point.
The rays of light converge after refraction converge at a point and that point is called as the principal force of the lens.