What does focal length measure?

Focal length is a fundamental concept in optics that specifically measures the distance from the lens to the focal point. The focal point is where parallel rays of light converge after passing through the lens. In a converging lens, this point is on the side opposite the incoming light. The focal length determines how strongly the lens converges or diverges light; a shorter focal length means the lens has a greater curvature and can bend light more sharply, while a longer focal length indicates a gentler curve.

Understanding the relationship between focal length and the way lenses operate is crucial for designing optical systems, such as cameras and telescopes. It directly impacts the size of the image formed and its distance from the lens. The other options, while they may describe relevant aspects of optics, do not accurately define what focal length specifically measures.