The point at which, after passing through the collecting lens, a beam of light parallel to the main optical axis is focused is called the main focus of the lens. After passing a beam parallel to the main optical axis through the diffusing lens, the light appears to come from a point behind the lens, which is called the imaginary main focus of the diffusing lens.
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.