Image Formation by a Concave Mirror

To find where a mirror forms an image, you don't need to trace every ray from an object — just two are enough, because any two reflected rays will cross at the image point. Three handy rays to pick from: a ray parallel to the principal axis (reflects through F), a ray through F (reflects out parallel to the axis), and a ray through C (reflects straight back along the same path).

For a concave mirror, the image completely transforms depending on where the object is. Far away (beyond C), the image is small, real, and inverted, sitting between F and C. Move the object to exactly C, and the image is the same size, still real and inverted, also at C. Bring it between C and F, and the image grows larger than the object, real, inverted, and now beyond C.

Right at F, something interesting happens: the reflected rays become parallel and never meet — no image forms at all (or forms at infinity). Move the object even closer, between F and the mirror, and everything flips: the image becomes virtual, erect, and magnified, appearing to sit behind the mirror. This is exactly how a shaving or dentist's mirror works.