Knowing the focal length of a lens is crucial in optics because it determines the lens’s optical characteristics and behavior. The focal length defines where parallel rays either converge or appear to diverge after passing through the lens. This information is fundamental for understanding image formation, magnification, and the behavior of light in optical systems. It is vital in designing and optimizing various optical instruments, such as cameras, telescopes, and eyeglasses. Additionally, the focal length influences the power of a lens and its ability to focus light, providing essential parameters for precise optical calculations and ensuring the effectiveness of optical devices in various applications.