What kind of lens is called thin?

A thin lens is a lens whose thickness is much less than its focal length. If the lens cannot be considered thin, then each of the two spherical surfaces of the lens can be considered as a separate thin 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.