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A plane mirror sends each incident ray into its specularly reflected direction, with equal incidence and reflection angles measured from the surface normal. For a point object in front of the mirror, the backward extensions of the reflected rays meet at the reflected point behind the plane. The image is therefore virtual, upright, equal in size and as far behind the mirror as the object is in front. The actual rays do not travel behind the mirror. In coordinates with mirror plane $x=0$, the object point $(x,y,z)$ is mapped to $(-x,y,z)$.

A converging convex lens refracts a paraxial pencil according to the thin-lens equationwhere $u>0$ is the distance to a real object, $f>0$ is the focal length, and $v>0$ denotes an image on the opposite side. Two constructed rays locate the image: a ray parallel to the optical axis emerges towards the far optical focus, and a ray through the ideal thin lens's optical centre proceeds undeviated. Their intersection is a real image; if they diverge, their backward extensions locate a virtual one.

For $u>2f$ the real inverted image is reduced; at $u=2f$ it is equal in size; for $f<u<2f$ it is enlarged. At $u=f$ the emergent pencil is parallel and the image is at infinity in the ideal model. For $0<u<f$, the image is virtual, upright and enlarged on the object's side. A distant object gives an image near the far focal plane. The lensmaker's equation explains why shape alone does not guarantee convergence: the surrounding medium and refractive index also matter. Spherical aberration, chromatic aberration and diffraction limit the ideal point-image construction. The statement that the centre ray is undeviated is the thin-lens approximation, rather than a general rule for a thick compound optical instrument.