Have negative focal lengths
Diverge collimated incident light
Forms only virtual images which are seen through the lens
Reduce spherical aberration, coma and distortion at negative-infinite or near-infinite conjugate ratios
With the concave surface facing the longest conjugate distance
These lenses are thicker at the edge than in the centre and flat on one side. The plano-concave lenses are used to expand light beams or to increase focal lengths in optical systems. They are often employed for beam expansion of high peak power pulsed lasers. A beam incident on a concave surface will be focused to a point outside the instrument.