What does the term 'focal length' refer to in optics?

The term 'focal length' in optics is defined as the distance from the lens to its focal point. This focal point is where light rays that are originally parallel to the optical axis converge after passing through the lens. The focal length is a crucial property of a lens, as it helps to determine the lens's ability to converge or diverge light. A longer focal length means the lens is less curved, resulting in an image that is farther away from the lens, while a shorter focal length indicates a more curved lens that brings light to a focal point closer to the lens itself. This characteristic is fundamental in designing optical systems, such as cameras, microscopes, and telescopes, where precise control over image formation is required.

In contrast, the distance from the lens to the eye does not describe the focal length, nor does the thickness of the lens or the diameter of the lens. These characteristics pertain to the physical dimensions and geometry of the lens rather than its optical properties related to focal length.