If the lens is plano-concave, a collimated beam of light passing through the lens is diverged (spread); the plano-concave lens is thus called a negative or diverging lens. The beam after passing through the lens appears to be emanating from a particular point on the axis in front of the lens; the distance from this point to the lens is also known as the focal length, although it is negative with respect to the focal length of a converging lens.

BK7 Optical Glass Lenses, Plano-Concave Design
Diameter 1-inch, AR coated at 400-750nm