By Zencalcy Editorial TeamLast reviewed About us
The mirror equation relates the object distance, image distance and focal length of a spherical mirror. A positive image distance means a real image on the same side as the object; a negative one means a virtual image behind the mirror.
Image distance for a curved mirror.
1/f = 1/d_o + 1/d_i
f is the focal length, d_o the object distance and d_i the image distance. For a concave mirror f is positive; for a convex mirror it is negative.
| Symbol | Meaning |
|---|---|
| f | Focal length, positive for concave, negative for convex. |
| d_o | Object distance from the mirror. |
| d_i | Image distance; negative means virtual. |
1/d_i = 1/0.2 − 1/0.5 = 3, so d_i = 0.333 m. The image is real, on the object side, and the magnification is −0.667.
Continue with the Mirror Focal Length Calculator, Numerical Aperture Calculator, or the Optical Path Length Calculator — or browse more optics and light tools and all physics calculators.
A negative image distance means the image forms behind the mirror and cannot be projected on a screen. Convex mirrors and magnifying mirrors produce virtual images.
Results are computed from the standard equations of classical mechanics. Unit conversions use the SI definitions published by NIST. Air resistance is ignored unless the calculator names a drag model.
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Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026