True or false? a diverging lens is sometimes called a negative lens because we use a negative value for the focal length in the thin-lens equation.

Diverging lens diverges ray of light. Their focal length is taken as negative because according to sign convention, the radius of curvature of that surface on which light refracts is to the left and every number to the left of 0 in real number line is taken as negative. 

Pole is considered as origin. If focal point(F) lies to the left of pole, then it is taken as negative. Why? it is a rule