This tutorial explains what is meant by the focal length of a lens and two factors which affect it. The focal length is the distance from the lens to the principle focus of the lens (where parallel rays converge for a convex lens). The two factors which affect the focal length of a lens are: - The refractive index of the lens material - The curvature of the lens surface A high refractive index and/or a highly curved lens will give a lens with a short focal length (i.e. the lens will be very good at bending light rays to a focus). Check out more GCSE physics tutorials at https://gcsephysicsninja.com/lessons/waves/