Any lens that is "thinner in the center" than on the edges is called a concave lens and will function as a diverging lens when operating in air.
In this Worksheet we learn how light is refracted by double concave lens in a manner that a virtual image is formed. [list=1] [*] Any incident ray traveling parallel to the principal axis of a diverging lens will refract through the lens and travel in line with the focal point (i.e., in a direction such that its extension will pass through the focal point). [*] Any incident ray traveling towards the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis. [*] An incident ray that passes through the center of the lens will in affect continue in the same direction that it had when it entered the lens. [/list] These three rules will be used to construct ray diagrams. A ray diagram is a tool used to determine the location, size, orientation, and type of image formed by a lens. Now have a closer look on the worksheet! You can move the circles to change the size of the object and the distance to the lens. You can also variate the focal length. Write down your observations! What happens with the image?