Mirrors and lenses form images by redirecting families of rays. An image point is where rays from one object point actually meet or where their backward extensions appear to meet.

Real and virtual images

A real image forms where light rays physically converge and can be projected onto a screen.

A virtual image forms where rays diverge but appear to originate from a point when traced backward. It cannot be projected directly onto a screen.

Principal rays

For spherical mirrors and thin lenses, a small set of easily constructed rays can locate an image. Rays parallel to the principal axis, rays through or toward a focal point, and rays through the optical center or center of curvature are especially useful.

Image equation and magnification

Both ideal spherical mirrors and thin lenses use

$$\frac1f=\frac1{d_o}+\frac1{d_i},$$

with the sign convention appropriate to the optical element.

Magnification is

$$m=\frac{h_i}{h_o}=-\frac{d_i}{d_o}.$$

Physical interpretation

The formulas and ray diagrams describe the same geometry. A reliable analysis uses both: the diagram shows whether an image should be real or virtual and upright or inverted, while the equations determine its location and scale quantitatively.