What is the focal length of a convex lens?

Answer and explanation;

-The focal length is the distance from the center of the lens to the principal focus (or focal points) of the lens. For a converging lens (for example a convex lens), the focal length is positive. On the other hand, the focal length of concave lens is negative.

Thus, for convex lens, the focal length is a distance over which all the parallel rays will converge. A certain point over that distance all the parallel rays will converge, that point is known as focal point.