The optical center of the lens, the centers of the spherical surfaces bounding the lens. The straight line connecting the centers of the spherical surfaces and the optical center of the lens is called the main optical axis. All other straight lines passing through the optical center are called side optical axes. The point where all the rays are collected after refraction by the lens is called the main focus. Focal length is the distance from the center of the lens to the main focus. The plane perpendicular to the main optical axis and passing through the main focus of the lens is called the focal plane.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.