Concave polygon a convex polygon. As with any simple polygon, the sum of the internal angles of a concave polygon is ("n" − 2) radians, equivalently 180°×("n" − 2), where "n" is the number of sides. It is always possible to partition a concave polygon into a set of convex polygons. A polynomial-time algorithm for finding a decomposition into as few convex polygons as possible is described by . A triangle can never be concave, but there exist concave polygons with "n" sides for any "n" > 3. An example of a concave quadrilateral is the dart. At least one interior angle
Lens (optics) real object at the location of that virtual image. Likewise, it appears to a subsequent lens as if it were an object at that location, so that second lens could again focus that light into a real image, "S" then being measured from the virtual image location behind the first lens to the second lens. This is exactly what the eye does when looking through a magnifying glass. The magnifying glass creates a (magnified) virtual image behind the magnifying glass, but those rays are then re-imaged by the lens of the eye to create a "real image" on the retina.