What is defined as the measurement from the back surface of the lens to the point of focus of the light rays?

The measurement from the back surface of the lens to the point where light rays converge is referred to as the focal length. This measurement is critical in optics as it determines the lens's ability to bend light and focus it. The focal length is a property that influences how the lens forms images and affects the overall optical performance of the system.

When light rays pass through a lens, they are refracted; the point where these rays actually meet after passing through the lens is the focal point. The distance from this point to the back surface of the lens is what we quantify as the focal length. Understanding this concept is essential for applications in photography, vision correction, and various optical instruments, where precise focusing is necessary.