For mirror Sign in the case of concave mirror: *.Since, object is always placed in front of the mirror hence the sign of object is taken as negative. *.Since, the centre of curvature and focus lie in front of the concave mirror, so signs of radius of curvature and focal length are taken as negativein the case of concave mirror. *.When image is formed in front of the mirror, the distance of image is taken as – (negative) and when image is formed behind the mirror, the distance of image is taken as + (positive). *.Height of image is taken as positive in the case of erect image and taken as negative in the case of inverted image. Sign 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. *.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 mirror. *.In the case of convex mirror, image always formed behind the mirror, thus the distance of image is taken as positive. *.In the case of a convex mirror, always an erect image is formed, thus the height of image is taken as positive. For lenses , Sign convention for lens is similar to that of spherical mirror. Signs are taken left of theoptical centre as negative, right of the optical centre as positive, above of the principal axis as positive and below of the principal axis as negative. Sign Convention - The new sign convention is known as New Cartesian Sign Convention. In this sign is taken negative towards left and taken as positive towards right at X-axis from origin.The sign is taken as positive (+) above the origin point at Y-axis; and below the origin point as negative (-) at Y-axis.