CONCAVE

, an appellation used in speaking of the inner surface of hollow bodies, more especially of spherical or circular ones.

Concave glasses, lenses, and mirrors, have either one side or both sides concave.

The property of all concave lenses is, that the rays of light, in passing through them, are deflected, or made to recede from one another; as in convex lenses they are inflected towards each other; and that the more as the concavity or convexity has a small<*> radius. Hence parallel rays, as those of the sun, by passing through a concave lens, become diverging; diverging rays are made to diverge more; and converging rays are made either to converge less, or to become parallel, or go out diverging. And hence it is, that objects viewed through concave lenses, appear diminished; and the more so, as they are portions of less spheres. See Lens.

Concave mirrors have the contrary effect to lenses: they reflect the rays which fall on them, so as to make them approach more to, or recede from each other, than before, according to the situation of the object; and that the more as the concavity is greater, or as the radius of concavity is less. Hence it is that concave mirrors magnifying objects that are presented to them; and that in a greater proportion, as they are portions of greater spheres. And hence also concave mirrors have the effect of burning glasses. See Mirror, and Burning Glass.