Lens
Definition
A curved piece of glass or plastic that bends light to focus it — the heart of cameras, glasses, and microscopes.
Technical definition
A transmissive optical element with curved surfaces that refracts light to converge (convex) or diverge (concave) rays, characterized by its focal length through the lensmaker's equation and the thin-lens equation.
Example
Reading glasses are convex lenses that add the converging power aging eyes have lost — they pre-focus incoming light so the retina receives a sharp image of close text.
A lens is not a mirror, and covering half a lens does not give half an image. Every part of a lens contributes rays to every part of the image — block half the lens and the whole image simply dims. Students expect half an image because they picture rays like laser pointers; real image formation uses the full aperture.