Final answer:
The two properties of a lens that affect the focal length are the centers of curvature and the material the lens is made from.
Step-by-step explanation:
Lenses focus light by bending the rays of light that pass through them. The focal length of a lens is determined by two properties:
The centers of curvature of the two sides of the lens: The distance between the lens and its centers of curvature affects the focal length. A lens with a shorter distance between the centers of curvature has a shorter focal length, while a lens with a longer distance between the centers of curvature has a longer focal length.
The material the lens is made from: The refractive index of the lens material influences the focal length. Lenses made from materials with a higher refractive index have a shorter focal length, while lenses made from materials with a lower refractive index have a longer focal length.