lens

A lens is a piece of transparent material, usually glass or plastic, with at least one curved surface that bends light passing through it.

• A simple drop of water sitting on an oiled surface behaves exactly like a lens — its curved dome shape makes text underneath look larger, the same trick a magnifying glass performs.
• Unlike mirrors, which reflect light, lenses let light pass through them — which is why we say we see objects through a lens, never in one.
• Every lens is either convex (thicker in the middle) or concave (thicker at the edges), and that single difference in shape decides whether the lens converges or diverges the light passing through it.
• Lenses are everywhere once you start looking for them: eyeglasses, cameras, telescopes, microscopes, and even the human eye itself all rely on a lens to focus light.