Ray diagram for double convex and double concave lenses. Drag the point f or f' (which ever is light blue) to change the focal length of the lens. Drag the point E (light blue) to change the location (horizontal drag) and height (vertical drag) of the object. The thickness of the lens does not correspond to the focal length.
What phenomenon does the failure of all 3 lines to cross neatly at a single point demonstrate? When is the image right side up? When is the image real? When is the image larger than the object? What happens when you drag f through the lens to the other side? How is that related to the sign conventions for lenses?