Mirror formula

Here u is the distance of the object from the pole of the mirror. v is the distance of the image, and f is the focal length. The formula works for concave and convex mirrors, wherever the object stands.

It only works if every value carries the right sign. The signs come from the New Cartesian Sign Convention. The focal length is also half the radius of curvature, so f = R/2.

A small example

A convex mirror has a focal length of 20 cm. A car is 60 cm in front of it. The focus of a convex mirror is behind it, so f = +20 cm. The car is in front, so u = -60 cm.

• 1/v = 1/f - 1/u = 1/(+20 cm) - 1/(-60 cm).
• 1/v = 3/(60 cm) + 1/(60 cm) = 4/(60 cm).
• v = +15 cm.

The plus sign puts the image behind the mirror. So it is virtual and upright. A last line that says this in words finishes the answer.

Where marks go

CBSE's 2026-27 Science sample paper has no mirror numerical. Its lens numerical, Question 35, shows how such answers are marked. The 2026-27 Science marking scheme gives 1 mark for the image distance and a separate mark for the focal length. Each value found is a line that can earn its own mark.

So write the formula, the values with their signs and units, and then the answer. That is how step marking splits an answer.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

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