Lens Bench

Thin lens:
  1/v − 1/u = 1/f
  m = v/u
  (Cartesian: u negative when object is on left)

Try this: Convex lens (f = +20), slowly move u from −60 → −5 cm. Watch v go from just past F′ to −∞, jump to +∞ on the virtual side, then shrink back. That sign flip at u = −f is the boundary between real and virtual images.