Plano-concave lenses are always negative. They are used to diverge collimated incident light or to form virtual images which are seen by looking through the lens.
Plano-concave lenses are often used to increase focal lengths in existing optical systems.
An uncoated lens can reflect up to eight percent of the incident light. To reduce this reflection, antireflection coatings are often added to both lens surfaces. Most plano-concave lenses are available with a variety of antireflection coatings.
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