Which method can be used to alter a lens's focal length?

Altering the curvature of one surface of a lens is a direct method to change its focal length. The focal length of a lens is determined by the curvature of its surfaces and the refractive index of the lens material. When the curvature of one or both surfaces is increased (made more convex) or decreased (made more concave), the way light bends as it passes through the lens changes. This modification impacts how quickly the light converges to a point and thus alters the focal length.

For instance, a more curved lens will have a shorter focal length, allowing it to converge light rays more quickly, while a less curved lens will have a longer focal length, dispersing light over a greater distance. This principle is foundational in optical design and correction, making it crucial for optics, including prescription eyewear.

Changing the size of the lens, applying a colored coating, or using different types of materials does not effectively change the focal length in the same way. While lenses of different sizes or materials might affect factors like weight or durability, they do not inherently alter how the lens focuses light, which is primarily governed by curvature and refractive index.