This lenses can be converging or diverging lenses. A thin lens has two focal points, located on the optical axis, a distance f from the center of the lens on either side of the lens. Parallel rays passing through a thin converging lens come together at the focus f on the opposite side of the lens, and parallel rays are passing through a diverging lens diverge as if they were leaving the focus on the incident side on a straight line path.
Thin lenses can form real and virtual image.
Let x o denote the perpendicular distance of the object from the centerline of the lens and let x o be positive. Let x i denote the perpendicular distance of the image from the centerline of the lens. If M is negative, the image is inverted. We can find the position and size of the image geometrically. Only two rays must be drawn.