It is convenient to use three beams to build an image in a thin lens:
1) The beam passing through the main focus of the lens F after refraction in the lens propagates parallel (along) the main optical axis.
2) A beam parallel to the main optical axis, after refraction in the lens, passes through the main focus F of the lens.
3) The beam passing through the optical center of the lens is not refracted.
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.