Give the important rules of the new cartesian sign convention followed for reflection of light by spherical mirrors.

New cartesian sign convention:

The pole P of the spherical mirror is taken as the origin and the principal axis of the mirror is along the X-axis of the coordinate system. 

Keeping this in mind, we can easily note the following facts: 

(i) Object distance, u is always taken negative, because the object is placed on the left of the mirror. 

(ii) In case of a concave mirror, a real image is formed in front (left side) of the mirror and a virtual image is formed behind (right side) the mirror, so image distance v is negative for a real image and positive for a virtual image. 

(iii) In case of a convex mirror, the image distance v is always positive because the image is formed behind the mirror and is virtua. 

(iv) The object height h is always taken positive, because the object is placed above the principal axis. 

(v) The image height h' is taken negative for real image (as it lies below the principal axis) and the image height h' is taken positive for a virtual image (as it lies above the principal axis). 

(vi) The focal length and radius of curvature of a concave mirror are taken negative because the principal focus lies on the left of the mirror. 

(vii) The focal length and radius of curvature of a convex mirror are taken positive, because the principal focus lies on the right hand side of the mirror.