what is the new cartesian sign conventions of concave and convex lens separately

New Cartesian sign convention for refraction of light through spherical lenses (i.e. both convex and concave lens):

According to this sign convention:

(i) All distances are measured from the optical centre of the lens.

(ii) The distances measured in the same direction as the incident light are taken positive.

(iii) The distances measured in the direction opposite to the direction of incident light are taken negative.

(iv) Heights measured upwards and perpendicular to the principal axis are taken positive.

(v) Heights measured downwards and perpendicular to the principal axis are taken negative. 

 

 Consequences of new Cartesian sign convention:

(i)The focal length of a convex lens is positive and that of a concave lens is negative.

 (ii)Object distance u is always negative.

 (iii)The distance of real image is positive and that of virtual image is negative.

(iv)The object height h is always positive. Height h' of virtual erect image is positive and that of real inverted image is negative.

(v)The linear magnification, m = h'/h is positive for a virtual image and negative for a real image.