Class 10 Mirror and Lens Sign Conventions Compared

Compare the rules

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Read the labelled model

Assign signs from directions

Distances measured in the direction of incident light are positive; those measured oppositely are negative. Heights above the principal axis are positive and those below it negative. These are directed quantities. Do not decide that every real image must have a positive image distance.

A concave mirror forming a real image places it on the object side, giving negative v in this arrangement. A convex lens forming a real image places it on the opposite side, giving positive v. Both images can be inverted.

Compare two original calculations

For a concave mirror with f = −12 cm and u = −36 cm, 1/v = −1/12 + 1/36 = −1/18. Thus v = −18 cm and m = −(−18)/(−36) = −0.5. The image is real, inverted and half the object height.

For a convex lens with f = +12 cm and u = −36 cm, 1/v = 1/12 − 1/36 = 1/18. Hence v = +18 cm and m = 18/(−36) = −0.5. The size and orientation agree, but the image lies on the other side of the lens.

A final three-part check

State position, orientation and relative size separately. Negative magnification signals inversion; its magnitude gives the size ratio. If your sketch shows a real image on the opposite side of a mirror, revisit the physical setup before changing signs to force an answer.

Choose what to study next

Sources and scope

This is a focused learning resource, not a complete chapter or a prediction of board questions. Use your current syllabus and the paper-specific marking scheme for assessment requirements.

Try a short check, then return to the explanation

Optional TutorMax original practice: five questions on light, lenses and mirrors, with explanations and a fresh retest. These checks sample a few skills; they are not official paper questions or a whole-subject readiness score. Existing explanations and official paper downloads stay available on this page.