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Biconcave lenses have negative focal lengths and form virtual images, which are seen by looking through the lens.
The most common biconcave lens is the symmetric-concave (aka equiconcave) lens, which has an identical concave surface on both sides of the lens. They are often used to expand light or to increase the focal lengths of existing systems. They are used in laser beam expanders, optical character readers, viewers, and projection 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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