For virtual image it will always be positive and for real image it will always be negative for mirror and image distance will always be negative in both the case and for lens f of convex lens will be + and f of concave lens is -

Sign Convention for Lenses and Mirrors



In optics, the sign convention is a set of rules used to determine the sign (positive or negative) of the distances, heights, and focal lengths associated with lenses and mirrors. It helps to maintain consistency and simplify calculations in optical systems. The sign convention for both lenses and mirrors follows a similar set of rules.

• Distances to the left of the lens are considered negative, while distances to the right are positive.


• Heights above the principal axis are positive, and heights below the principal axis are negative.

• For a converging lens (convex), the focal length is positive.


• For a diverging lens (concave), the focal length is negative.

• Real images are formed on the opposite side of the lens from the object and have positive image distances.


• Virtual images are formed on the same side as the object and have negative image distances.

• Distances in front of a mirror (to the left) are considered positive, while distances behind the mirror (to the right) are negative.


• Heights above the principal axis are positive, and heights below the principal axis are negative.

• For a concave mirror (converging), the focal length is negative.


• For a convex mirror (diverging), the focal length is positive.

• Real images are formed in front of the mirror, on the opposite side of the object, and have positive image distances.


• Virtual images are formed behind the mirror, on the same side as the object, and have negative image distances.

In summary, the sign convention for both lenses and mirrors follows similar rules. Distances and heights are positive or negative based on their position with respect to the lens or mirror. Focal lengths are positive for converging lenses and concave mirrors, while they are negative for diverging lenses and convex mirrors. Image formation is determined by whether the image is real or virtual, with positive image distances for real images and negative image distances for virtual images. Understanding and applying the sign convention correctly is crucial for performing accurate calculations and analysis in optics.