Final answer:
The focal length of a lens is the distance from the center of the lens to its focal point, where light rays converge or appear to diverge. It is denoted by the symbol f.
Step-by-step explanation:
The focal length of a lens is the distance from the center of the lens to its focal point. It is denoted by the symbol f. The focal point is the point where light rays parallel to the principal axis converge or appear to diverge from after passing through the lens.