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Concave Lens
A concave lens is curved inward; it is shaped like two dishes placed back-to-back. Light passing through a concave lens bends outward, or diverges. Unlike convex lenses, which produce real images, concave lenses produce only virtual images. A virtual image is one perceived by the visual portion of the brain and appears as a smaller image just in front of the actual object (in this case a shamrock). Concave lenses are generally prescribed for myopic, or near-sighted, people. Concave lenses help the eyes to produce a focused image on the retina instead of in front of it.
From Principles Behind Lenses
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