Mirror
Definition
A smooth reflective surface that forms images by bouncing light — flat mirrors show you yourself; curved mirrors magnify or shrink the world.
Technical definition
An optical element that forms images by specular reflection. Plane mirrors give upright, same-size virtual images; spherical mirrors follow 1/f = 1/d_o + 1/d_i with R = 2f, producing real or virtual images depending on object distance.
Example
A car's passenger-side mirror is convex: it shrinks the image to squeeze a wide view into a small glass — which is exactly why 'objects in mirror are closer than they appear.'
Mirrors do not flip left and right — they flip front and back. Raise your right hand and the mirror image raises its left, but that is because the mirror reverses depth, not because it rotates the image. The real test: text held to a mirror reads backwards because front-to-back reversal turns the page around, like pressing ink onto blotting paper.