concave lens

A concave lens is a lens that's thicker at its edges than in the middle.

• Whatever the distance of the object behind it, a concave lens always produces an image that's erect and diminished in size — simpler and more predictable behaviour than a convex lens's.
• A concave lens spreads out (diverges) parallel beams of light passing through it, earning it the alternate name "diverging lens" — the exact opposite effect of a convex lens.
• Because it only ever diminishes images rather than magnifying them, a concave lens is used in eyeglasses for people who are near-sighted, spreading out light before it reaches the eye to correct where the image would otherwise focus.
• Comparing a concave lens against a flat glass plate makes its effect obvious: parallel light passes straight through a flat plate unchanged, while the same beams visibly fan outward after passing through a concave lens.