Instead of a concave lens near the eye, Kepler used a convex lens. The result was that ... Other scientists had speculated that using mirrors rather than lenses might correct this problem, but ...

In a ray diagram, a concave lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens.

French photography pioneer Charles Nègre created a 3D self-portrait using a "witch's mirror" - a clever piece of 19th-century ...

In a ray diagram, a concave lens is drawn as a vertical line with inward facing arrows to indicate the shape of the lens.