According to cartesian sign convention :
1) All distance are measured from the pole of the mirror ( or ) the optical centre of the lens.
2) The distance measured in the same direction as incident light are taken as positive.
3) The distance measured in the direction opposite to the incident light are taken as negative.
4) The heights measured upwards with respect to x-axis and normal to the x-axis are taken as positive.
5) The heights measured downwards are taken as negative.
