![optics - How is focal length defined for a two-lens system, separated by a distance $d$? - Physics Stack Exchange optics - How is focal length defined for a two-lens system, separated by a distance $d$? - Physics Stack Exchange](https://i.stack.imgur.com/fwrwB.png)
optics - How is focal length defined for a two-lens system, separated by a distance $d$? - Physics Stack Exchange
![optics - What happens in a two lens system with two converging lenses when the object is placed at the focus of the first lens? - Physics Stack Exchange optics - What happens in a two lens system with two converging lenses when the object is placed at the focus of the first lens? - Physics Stack Exchange](https://i.stack.imgur.com/yhbOs.png)
optics - What happens in a two lens system with two converging lenses when the object is placed at the focus of the first lens? - Physics Stack Exchange
![Two-lens system in the figure, O (the object) stands on the common central axis of two thin, symmetrical lenses, which are mounted in the boxed regions. Lens 1 is mounted within the Two-lens system in the figure, O (the object) stands on the common central axis of two thin, symmetrical lenses, which are mounted in the boxed regions. Lens 1 is mounted within the](https://homework.study.com/cimages/multimages/16/hhhhhh8211941138878291129.jpg)