Since the image is virtual, we need to assign a negative sign for the image distance. A light source is located to the left of a lens. Formation of a real image is also shown. Click-Drag the lens until the refracted rays are parallel. At this point the source is located at the focal point.
Focal length is the distance from the focal point to the lens.
An object is placed to the left of a concave lens and the formation of the virtual image can be seen. The image is virtual or non-real here, because, one has to extend the rays to locate the position of the image. Since the image is virtual, we need to assign a negative sign for the image distance. Put the convex lens in the lens holder and place it in the optical bench so that the object distance is 5 cm. Move the screen and see whether you get a real image on the screen.