Which method can alter the focal length of a lens?

Altering the curvature of one side of a lens directly affects its focal length due to the principles of optics. When the curvature is changed, it alters the way light passes through the lens. A steeper curve results in a stronger lens, which means it has a shorter focal length, whereas a flatter curve produces a weaker lens with a longer focal length. The focal length is notably influenced by the radius of curvature as described by the lens maker's formula, which accounts for the refractive index and curvature of the lens surfaces. Therefore, by modifying the curvature, you can effectively change the lens's ability to converge or diverge light, thereby adjusting the focal point.

Increasing the lens diameter does not significantly change the focal length. It may allow more light to pass through but does not alter the way light is refracted by the lens surfaces. Similarly, adding a tint to the lens generally affects the color and tint density but does not impact the focal properties of the lens. Repositioning the lens in the frame may change the position of the lens relative to the wearer's eyes, but it does not affect the intrinsic focal characteristics of the lens itself. Hence, altering the curvature remains the only method among the options presented that directly impacts the focal length.