Spherical-mirror images and signs
A virtual image is formed by ray extensions, not actual ray convergence.
Key knowledge
In paraxial spherical-mirror optics, 1/f=1/d_o+1/d_i, f=R/2 and transverse magnification m=−d_i/d_o. In the real-is-positive convention, concave f,R are positive, convex f,R negative; real images have d_i>0 and virtual images d_i<0. Negative m inverts the image. Outside the paraxial region, spherical aberration prevents one exact focus.
Values and equations
1/f=1/d_o+1/d_i; f=R/2; m=−d_i/d_o
- f,R,d_o,d_i
- Signed focal length, curvature radius and object/image distances m
- m
- Signed transverse magnification 1
Conditions and scope
- Small ray angles and heights compared with |R|; consistent real-is-positive object/image signs and nonzero finite distances in the displayed expressions.
- A plane mirror is the R→∞ limit and has d_i=−d_o. Other sign conventions are valid when used consistently; a virtual image does not require light to travel behind an opaque mirror.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Read §2.2 paraxial focal relation2.1, mirror equation2.6, explicit sign convention and magnification2.8. Restore paraxial scope rather than adopting prose that all parallel rays of a spherical mirror share one focus.
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