Focal length is a key optical parameter that defines the distance from the optical center of a lens or mirror to its focal point. It is a measure of how strongly the lens converges or diverges light. In converging lenses (convex), where light rays come together, the focal length is positive. In diverging lenses (concave), where light rays appear to diverge, the focal length is negative. The focal length influences the magnification and field of view in optical systems. Shorter focal lengths result in wider fields of view, while longer focal lengths lead to magnified images. Focal length is a crucial factor in photography, astronomy, and various optical applications, influencing the composition and characteristics of images.