Sign Conventions. Sign Convention for Spherical Lenses : For lenses, we follow sign conventions, similar to the one used forspherical mirrors. The sign conventions for the given quantities in the lens equation and magnification equations are as follows: f is + if the lens is a double convex lens (converging lens) f is - if the lens is a double concave lens (diverging lens) d i is + if the image is a real image and located on the opposite side of the lens. The new Cartesian sign convention is same as that for spherical mirrors. According to the sign convention, the focal length of a convex lens is positive and that of a concave lens is negative. We can see the sign convention in both Lens Some points to note Focal length of Convex Lens is positive Focal length of Concave Lens is negative Since object is always in front of the lens, object distance is always negative If a real image is formed, image is formed on right side, so image distance is positive According to it: ... A convex lens of focal length 0.12 m produces a virtual n image which is thrice the size of the object. Sign convention is a set of rules to set signs for image distance, object distance, focal length, etc for mathematical analysis of image formation. Since, the centre of curvature and focus lies behind the convex mirror, so sign of radius of curvature and focal length are taken as + (positive) in the case of convex … All distances are measured from the optical centre of the mirror. For a Convex Lens, object can be kept at different positionsHence, we take different casesCase 1 - Object is Placed at infinityIn this Case, Object is kept far away from lens (almost at infinite distance)So, we draw rays parallel to principal axisSince ray parallel to principal axis passes through t Sign convention in the case of a convex mirror: Since, object is always placed in front of the mirror hence the sign of object is taken as negative. Sign convention - definition Sign convention is a set of rules to set signs for image distance, object distance, focal length, etc for mathematical analysis of image formation. According to the convention, the focal length of a convex lens is positive and that ofa concave lens is negative. ll A measurements are taken from the optical centre of the lens which lies on the origin of x-y axis. According to it: Object is always placed to the left of lens. Find the distance between the object and the lens. Sign convention for spherical lenses. The difference is that the distances are measured from the optical centre.