Define the following :(i) Focal length.(ii) Principal focus

(i) Focal Length: The focal length of a lens is the distance between the lens and its focus. It is usually represented by the letter 'f'. The focal length is determined when the lens is focused at infinity. The light rays that pass through the lens will converge or diverge at the focus point.

(ii) Principal Focus: The principal focus of a lens is the point where light rays parallel to the axis converge after passing through the lens, or appear to diverge from. For a converging lens (like a convex lens), the principal focus is the point where the light rays meet. For a diverging lens (like a concave lens), the principal focus is the point from where the light rays appear to be coming.